EDUCATION  DEPT, 


7> 


POPULAR 


PHYSICS 


BY 


J    DORMAN   STEELE,  PH.D.,  F.G.S. 

J  II 

AUTHOR   OF   FOURTEEN-WEEKS   SERIES   IN   NATURAL   SCIENCK 


The  works  of  God  are  fair  for  naught. 

Unless  our  eyes,  in  seeing, 
See  hidden  in  the  thing  the  thought 

That  animates  its  being. 


NEW   YORK     :    CINCINNATI     :    CHICAGO 

AMERICAN,  BQPK     COMPANY 


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EDUCATION  DEPT. 


Copyright,  1888,  by  A.  S.  BARNES  &  Co. 
Pop.  Phys. 

flrintefr  b$ 

.   8.  36arn:0  &  Company 


AUTHOR'S    PREFACE. 


work  has  grown  up  in  the  class-room.  It 
_!_  contains  those  definitions,  illustrations,  and  ap- 
plications which  seemed  at  the  time  to  interest  and 
instruct  the  author's  pupils.  "Whenever  any  explana- 
tions fixed  the  attention  of  the  learner,  it  was  laid 
aside  for  future  use.  Thus,  by  steady  accretions,  the 
process  has  gone  on  until  a  book  is  the  result. 

As  Physics  is  generally  the  first  branch  of  Natural 
Science  pursued  in  schools,  it  is  important  that  the 
beginner  should  not  be  wearied  by  the  abstractions 
of  the  subject,  and  so  lose  interest  in  it  at  the  very 
start.  The  author  has  therefore  endeavored  to  use 
such  simple  language  and  practical  illustrations  as 
will  attract  the  learner,  while  he  is  at  once  led  out 
into  real  life.  From  the  multitude  of  philosophical 
principles,  only  those  have  been  selected  which  are 
essential  to  the  information  of  every  well-read  person. 
Withiri  the  limits  of  a  small  text-book,  no  subject 
can  be  exhaustively  treated.  This  is,  however,  of 
less  importance  now,  when  every  teacher  feels  that 
he  must  of  necessity  be  above  and  beyond  any 
school-work  in  the  fullness  of  his  information.  The 
object  of  an  elementary  work  is  not  to  advance  the 
peculiar  ideas  of  any  person,  but  simply  to  state  the 
currently-accepted  facts  and  theories.  The  time-hon- 

M187542 


Vl  AUTHOR'S     PKEFACE. 

ored  classifications  recognized  in  all  scientific  works, 
have  been  retained.  In  order  to  familiarize  the 
pupil  with  the  metric  system,  now  generally  used 
by  scientific  men,  it  is  continually  employed  in  the' 
problems.  The  notes  contain  many  illustrations  and 
additional  suggestions,  but  their  great  value  will  ap- 
pear in  the  descriptions  of  simple  experiments  which 
are  within  the  reach  of  any  pupil. 

New  plates  being  required  for  this  edition,  the 
author  has  taken  the  opportunity  thoroughly  to  re- 
vise the  entire  work.  By  carefully  comparing  the 
criticisms  of  teachers,  he  has  tried  to  obtain  the 
"parallax"  of  all  its  statements  and  methods,  and 
to  eliminate,  as  far  as  possible,  the  errors  growing 
out  of  his  "personal  equation."  Hearty  thanks  are 
tendered  to  the  many  friends  of  the  book  who,  by 
their  suggestions  and  criticisms,  have  so  greatly 
added  to  the  value  of  this  revision.  To  name  them 
all  in  this  Preface  would  be  impossible,  and  to  dis- 
criminate would  be  invidious.  The  author  can  not, 
however,  allow  the  opportunity  to  pass  without  ex- 
pressing his  profound  sense  of  obligation.  By  untir- 
ing study  and  the  continued  help  of  his  friends,  he 
hopes  thus,  year  by  year,  to  make  the  series  more 
and  more  worthy  the  favor  which  his  fellow-teachers 
have  so  abundantly  bestowed  upon  it.  Happy  indeed 
will  he  be  if  he  succeed  in  leading  some  young  mind 
to  become  a  lover  and  an  interpreter  of  Nature,  and 
thus  come  at  last  to  see  that  Nature  herself  is  but 
a  "thought  of  God." 


PUBLISHERS'  PREFACE. 


series  of  elementary  text-books  in  science, 
-L  written  by  the  late  Dr.  J.  Dorman  Steele,  at- 
tained an  extraordinary  degree  of  popularity,  due  to 
the  author's  attractive  style,  his  great  skill  in  the 
selection  of  material  suited  to  the  demands  of  the 
schools  for  which  the  books  were  intended,  his  sym- 
pathetic spirit  toward  both  teachers  and  pupils,  and 
his  earnest  Christian  character,  which  was  exhibited 
in  all  his  writing. 

Shortly  before  his  death,  finding  his  health  too 
feeble  to  permit  of  extra  labor,  the  author  requested 
Dr.  W.  Le  G.  Stevens,  Professor  of  Physics  in  the 
Packer  Collegiate  Institute,  Brooklyn,  to  revise  the 
text-book  in  Physics,  as  important  advances  in  this 
department  of  science  had  been  made  since  the  issue 
of  the  edition  of  1878.  In  performing  this  work, 
Professor  Stevens  has  endeavored  to  impose  the  least 
possible  modification  upon  the  peculiar  style  of  the 
author.  Nevertheless,  every  chapter  has  received 
some  alterations  and  slight  enlargement.  In  a  work 
intended  for  higher  classes,  the  reviser  would  natu- 
rally make  the  treatment  of  every  subject  more  thor- 
ough ;  but  this  would  unfit  the  present  book  for  the 


PUBL1J3HEKS'     PREFACE. 

schools  to  which  it  was  originally  adapted.  It  is 
difficult,  moreover,  to  combine  a  strictly  popular  style 
with  that  precision  which  is  demanded  by  advanced 
students  in  exact  science.  But  although  the  field  in 
a  rudimentary  text-book  is  limited,  it  is  thought  that 
there  are  no  important  errors  of  statement  in  the 
present  hand-book. 

In  order  to  distinguish  this  revision  from  the 
older  editions,  the  name  is  changed  to  "  Steele's  Pop- 
ular Physics." 


NOTICE. 

The  publishers  of  this  book  will  still  issue  the  former  edition,  known  as 
"Steele's  fourteen  Weeks  in  Physics ,"  for  classes  already  organized  and  for 
teachers  who  may  prefer  to  continue  its  use.  Any  book-seller  can  obtain  the 
former  edition  if  requested  to  do  so. 


SUGGESTIONS  TO  TEACHERS. 


OTUDENTS  are  expected  to  obtain  information  from  this 
vO  book,  without  the  aid  of  questions,  as  they  must  always 
do  in  their  general  reading.  When  the  subject  of  a  paragraph 
is  announced,  the  pupil  should  be  prepared  to  tell  all  he  knows 
about  it.  He  should  never  be  allowed  to  answer  a  question, 
except  it  be  a  short  definition,  in  the  language  of  the  book. 
The  diagrams  and  illustrations,  as  far  as  possible,  should  be 
drawn  upon  the  blackboard  and  explained.  Although  pupils 
may,  at  first,  manifest  an  unwillingness  to  do  this,  yet  in  a 
little  time  it  will  become  an  interesting  feature  of  the  recita- 
tion..  In  his  own  classes,  the  author  has  been  accustomed  to 
place  upon  the  blackboard  the  analysis  of  each  chapter  of  the 
book,  and  require  the  pupils  to  recite  from  that,  without  the 
interposition  of  questions,  except  such  as  were  necessary  to 
bring  out  the  topic  more  clearly,  or  to  throw  a  side  light  upon 
it.  Where  the  analysis"  given  in  the  book  does  not  include  all 
the  minor  points  of  the  lesson,  the  pupils  can  easily  supply  the 
omission.  The  "Practical  Questions"  given  at  the  close  of 
each  general  subject,  have  been  found  a  profitable  exercise  in 
awakening  inquiry  and  stimulating  thought.  They  may  be 
used  at  the  pleasure  of  the  instructor.  The  equations  con- 
tained in  the  text  are  designed  to  be  employed  in  the  solution 
of  the  problems. 

It  should  constantly  be  borne  in  mind  that,  as  far  as  possible, 
every  question  and  principle  should  be  submitted  to  Nature  for 
a  direct  answer  by  means  of  an  experiment.  Pupils  should  be 
encouraged  to  try  the  simple  illustrations  necessary.  The  stu- 
dent who  brings  in  a  bit  of  apparatus  made  by  himself,  does 
better  than  if  he  were  merely  to  memorize  pages  of  text. 


X  SUGGESTIONS     TO     TEACHERS. 

The  following  works,  to  which  the  author  acknowledges  his 
obligation  for  valuable  material,  will  be  useful  to  teacher  as 
well  as  pupil,  in  furnishing  additional  illustrations  and  in  eluci- 
dating difficult  subjects,  viz.:  Tait's  "Recent  Advances  on  Phys- 
ical Science";  Arnott's  "  Elements  of  Physics"  (7th  ed.);  Stewart's 
"Elementary  Physics,"  also  his  "Conservation  of  Energy,"  and 
"Treatise  on  Heat";  Atkinson's  "Deschanel's  Natural  Philoso- 
phy"; Lockyer's  "Guillemin's  Forces  of  Nature";  Herschel's 
"Introduction  to  the  Study  of  Physical  Science";  Tomlinson's 
"Introduction  to  the  Study  of  Natural  Philosophy";  Beale's 
"How  to  Work  with  the  Microscope";  Schellen's  "Spectrum 
Analysis";  Roscoe  on  "Spectrum  Analysis";  Lockyer's  "The 
Spectroscope,"  and  "Studies  in  Spectrum  Analysis";  Airy's 
"Geometrical  Optics";  Nugent's  "Optics";  "Chevreul  on  Col- 
ors"; Thomson  and  Tait's  "Natural  Philosophy";  Maxwell's 
"Electricity  and  Magnetism";  Silvanus  Thompson's  "Lessons 
in  Electricity  and  Magnetism";  Faraday's  "Forces  of  Matter"; 
Youmans'  "Correlation  of  Physical  Forces";  Maury's  "Physical 
Geography  of  the  Sea";  Atkinson's  "  Ganot's  Physics";  Silli- 
man's  "Physics";  Tyndall's  Lectures  on  Light,  Heat,  Sound,  Elec- 
tricity, also  his  "Forms  of  Water";  Snell's  "  Olmsted's  Philoso- 
phy "  (revised  edition) ;  Loomis'  "  Meteorology  ";  Miller's  "  Chem- 
ical Physics";  Urbanitzky's  " Electricity  in  the  Service  of  Man"; 
Cooke's  "Religion  and  Chemistry";  Daniell's  "Principles  of 
Physics";  Anthony  and  Brackett's  "Text-book  of  Physics,"  and 
also  numerous  works  named  in  the  "  Reading  References "  at 
the  close  of  each  general  division.  They  may  be  procured  of 
the  publishers  of  this  book.  The  pupil  should  continually  be 
impressed  with  the  thought  that  the  text-book  only  introduces 
him  to  a  subject,  which  he  should  seek  every  opportunity  to 
pursue  in  larger  works  and  in  treatises  on  special  topics. 

The  editor  will  be  pleased  to  correspond  with  teachers  con- 
cerning the  apparatus  for  the  performance  of  the  experiments, 
or  with  reference  to  any  of  the  "Practical  Questions." 


TABLE  OF  CONTENTS. 


PAGE 

I.— INTRODUCTION 1 

I.— GENERAL  DEFINITIONS 3 

II. — GENERAL  PROPERTIES  OF  MATTER       ...  6 

III. — SPECIFIC  PROPERTIES  OF  MATTER    ...  10 

II.— MOTION  AND  FORCE          ...        .        .        .19 

III.— ATTRACTION      .        .        .        .        ....         41 

I.— MOLECULAR  FORCES       .        .       *       ...    43 
II.— GRAVITATION      .       .       .  ,     .       .       ,       .        55 

IV.— ELEMENTS  OF  MACHINES       .        .        .        .        .79 

V.— PRESSURE   OF  LIQUIDS  AND  GASES     .        .99 

I.— HYDROSTATICS        .       •     *>       •       •       •       •  101 

II.— HYDRODYNAMICS        .       .       .       •       •       •      121 

III.— PNEUMATICS 129 

VI.— SOUND 151 

VII.— LIGHT        .       . .189 

VIII.— HEAT  241 


Xll  CONTENTS. 

PACE 

IX.— MAGNETISM    .       .       ,       .       .       .       .  .        .279 

X.— ELECTRICITY      .        .*     .        .       .       .  '.        .291 

XI.— APPENDIX       .            .    .        .       .       .       .  .        .355 

1.— QUESTIONS             .       .       ....  .       .      357 

2.— INDEX                       .  .       .  375 


I. 

INTRODUCTION. 


"  WE  have  no  reason  to  believe  that  the  sheep  or  the  dog,  or  indeed 
any  of  the  lower  animals,  feel  an  interest  in  the  laws  by  which  natural 
phenomena  are  regulated.  A  herd  may  be  terrified  by  a  thunder-storm ; 
birds  may  go  to  roost,  and  cattle  return  to  their  stalls  during  a  solar 
eclipse;  but  neither  birds  nor  cattle,  so  far  as  we  know,  ever  think  of 
inquiring  into  the  causes  of  these  things.  It  is  otherwise  with  man.  The 
presence  of  natural  objects,  the  occurrence  of  natural  events,  the  varied 
appearances  of  the  universe  in  which  he  dwells,  penetrate  beyond  his 
organs  of  sense,  and  appeal  to  an  inner  power  of  which  the  senses  are  the 
mere  instruments  and  excitants.  No  fact  is  to  him  either  final  or  original. 
He  can  not  limit  himself  to  the  contemplation  of  it  alone,  but  endeavors 
to  ascertain  its  position  in  a  series  to  which  the  constitution  of  his  mind 
assures  him  it  must  belong.  He  regards  all  that  he  witnesses  in  the  pres- 
ent as  the  afflux  and  sequence  of  something  that  has  gone  before,  and  as 
the  source  of  a  system  of  events  which  is  to  follow.  The  notion  of  spon- 
taneity, by  which  in  his  ruder  state  he  accounted  for  natural  events,  is 
abandoned ;  the  idea  that  nature  is  an  aggregate  of  independent  parts  also 
disappears,  as  the  connection  and  mutual  dependence  of  physical  powers 
become  more  and  more  manifest ;  until  he  is  finally  led  to  regard  Nature 
as  an  organic  whole,  as  a  body  each  of  whose  members  sympathizes  with 
the  rest,  changing,  it  is  true,  from  age  to  age,  but  without  any  real  break 
of  continuity,  or  interruption  of  the  fixed  relations  of  cause  and  effect." 

TYNDALL. 


ANALYSIS  OF  THE  INTRODUCTION. 


I.    GENERAL  DEFINI- 
TIONS. 


II.    GENERAL  PROPER- 
TIES OF  MATTER. 


III.    SPECIFIC  PROPER- 
TIES OF  MATTER. 


1.  Of  Matter,  Body,  and  Substance 

2.  General  and  Specific  Properties. 

3.  The  Atomic  Theory. 

4.  Physical  and  Chemical  Changes. 

5.  Energy. 

6.  Physical  and  Chemical  Forces. 

7.  Definition  of  Physics  and  Chem- 

istry. 

1.  Extension  and  its  Measurement. 

2.  Impenetrability. 

3.  Divisibility. 

4.  Porosity. 

5.  Indestructibility. 

1.  Ductility. 

2.  Malleability. 

3.  Tenacity. 


4.  Elasticity. 

5.  Hardness. 

6.  Brittleness. 


(1.)  Compression. 
(2.)  Expansion. 
(3.)  Torsion. 
(4.)  Flexure. 


INTRODUCTION. 


I.     GENERAL    DEFINITIONS. 

1.  Matter. — Whatever  occupies  space  is  called  mat- 
ter.   A  definite  portion  of  matter  is  termed  a  ~body. — 
Examples:  A  lake,  a  dew-drop,  a  quart  of  oil,  an  an- 
vil, a  pendulum.    A  particular  kind  of  matter  is  styled 
a  substance. — Examples:  Gold,  wood,  stone,  oxygen. 

2.  General  and  Specific   Properties, — A   general 
property  of  matter  is  a  quality  that  belongs  to  all  sub- 
stances.—  Example:  Extension.      A  specific  property 
is  one  which  distinguishes  particular  substances. — Ex- 
amples: The  yellow  color  of  gold,  the  brittleness  of 
glass,  the  sweetness  of  sugar.     These  properties  are 
so  distinctive  that  we  say,  " yellow  as  gold,"  "brittle 
as  glass,"  "sweet  as  sugar." 

3.  The  Atomic  Theory  supposes 

(1.)  That  the  smallest  particle  of  matter  we  can 
see  is  composed  of  still  smaller  particles  or  molecules 
(tiny  masses),*  each  possessing  the  specific  properties 
of  the  substance  to  which  it  belongs. 

*  A  molecule  is  a  group  of  atoms  held  together  by  chemical  force,  and 
is  the  smallest  particle  of  a  substance  which  can  exist  by  itself.  Even  in 
a  simple  substance,  i. «.,  one  in  which  the  atoms  are  all  of  one  kind,  it  is 
thought  that  they  are  generally  clustered  in  molecules.  (See  "Chemistry," 
p.  4.)  In  water,  the  molecules  are  the  small  masses  which,  when  driven 


••VI::    :  A  INTRODUCTION. 

(2.)  That  each  molecule  consists  of  still  smaller 
portions,  called  atoms,*  which  are  regarded  as  indi- 
visible and  unchangeable. — Examples :  A  molecule  of 
water  is  made  up  of  two  atoms  of  hydrogen  and 
one  of  oxygen.  A  molecule  of  salt  consists  of  one 
atom  of  chlorine  and  one  of  sodium.  The  smallest 
piece  of  salt  contains  many  molecules.  By  dissolving 
in  water,  we  divide  it  into  its  separate  molecules,  and 
the  solution  has  a  briny  taste,  because  each  one  pos- 
sesses the  savor  of  salt. 

4.  Physical  and  Chemical  Changes. — A  physical 
change  is  one  that  does  not  affect  the  composition 
of  the  molecule,  and  so  does  not  alter  the  specific 
properties  of  a  substance. — Examples :  The  falling  of 
a  stone,  the  dissolving  of  sugar  in  water.    A  chemical 
change   is  one  that   implies  the   re-arrangement   of 
the  atoms  into  new  molecules  and  so  destroys  the 
specific  properties  of  a  substance. — Examples:  The 
rusting  of  iron,  the  burning  of  coal. 

5.  Energy. -7- The  power  of  producing  change   of 
any  kind  is  called  energy.    Heat,  light,  electricity,  etc., 

apart,  form  steam.  In  a  gas,  they  move  about  with  great  velocity,  colliding 
with  one  another  and  with  the  sides  of  the  containing  vessel.  The  collisions 
of  the  molecules  with  the  walls  of  the  vessel  account  for  the  pressure  which 
the  gas  exerts  upon  them. 

*  Animalcules  furnish  a  striking  illustration  of  the  minuteness  of 
atoms.  In  the  drop  of  stagnant  water  that  clings  to  the  point  of  a  needle, 
swarming  legions  swim  as  in  an  ocean,  full  of  life,  frisking,  preying  upon 
one  another,  waging  war,  and  re-enacting  the  scenes  of  the  great  world 
about  them.  These  tiny  animals  possess  organs  of  digestion  and  assimila- 
tion. Their  food,  coursing  in  channels  more  minute  than  we  can  conceive, 
may  be  composed  of  solid  as  well  as  liquid  matter;  and  finally,  at  the 
lowest  extreme  of  this  descending  series,  we  come  to  the  atoms  of  which 
the  matter  itself  is  composed.  The  most  powerful  of  microscopes  fails  com- 
pletely to  reveal  the  separate  molecule. 


GENERAL     DEFINITIONS.  5 

are  different  forms  of  energy.  Each  is  capable  of 
producing  change  in  matter. — Examples:  Heat  melts 
ice  ;  light  blackens  silver  chloride ;  a  wire  becomes  hot 
when  an  electric  current  passes  through  it.  The  same 
energy  may  be  manifested  successively  in  different 
forms. — Examples:  Heat  is  converted  by  the  steam- 
engine  into  motion ;  this  motion  imparted  to  a  dynamo 
is  converted  into  electricity,  which  may  be  further 
transformed  into  light,  heat  and  magnetism. 

6.  Physical  and  Chemical  Forces. — When  energy 
is  manifested  as  an  attraction  or  repulsion  we  speak  of 
it  as  a  forcCj  and  we  distinguish  between  physical  and 
chemical  forces  as  the  changes  produced  by  them  are 
physical  or  chemical. — Examples :  The  attraction  of 
an  electrified  body  for  small  bits  of  paper  is  a  physical 
force,  and  so  also  the  attraction  of  a  magnet  for  one 
end  of  a  compass-needle,  or  the  repulsion  for  the  other 
end.    The  attraction  of  iron  for  oxygen  in  the  forma- 
tion of  iron-rust  is  a  chemical  force. 

7.  Physics  and  Chemistry. — The  former  treats  of 
physical  changes,  the  latter  of  chemical  changes  in 
matter.    The  unit  of  the  physicist  is  the  molecule,  of 
the  chemist  the  atom. 


PRACTICAL   QUESTIONS. 

1.  Name  some  specific  property  of  coal;  ink;  chalk;  grass;  tobacco; 
snow. 

2.  My  knife-blade  is  magnetized,  so  that  it  will  pick  up  a  needle ;  is 
that  a  physical  or  a  chemical  change? 

3.  Is  it  treated  in  Physics  or  Chemistry? 

4.  Is  the  burning  of  coal  a  physical  or  a  chemical  change? 
6.  The  production  of  steam?   The  formation  of  dew? 


INTRODUCTION. 

6.  The  falling  of  a  stone?    The  growth  of  a  tree? 

7.  The  flying  of  a  kite?     The  chopping  of  wood? 

8.  The  explosion  of  powder?     The  boiling  of  water? 

9.  The  melting  of  iron?    The  drying  of  clothes? 

10.  The  freezing  of  water?    The  dissolving  of  sugar? 

11.  The  forging  of  a  nail?    The  making  of  bread? 

12.  The  sprouting  of  a  seed?    The  decay  of  vegetables? 
13*  The  condensation  of  steam? 


II.    GENERAL   PROPERTIES  OF   MATTER. 

THERE  are  two  essential  properties  without  which, 
matter  is  inconceivable.  These  are  extension  and 
impenetrability. 

1.  Extension  is  the  property  of  occupying  space. 
The  amount  of  space  a  body  occupies  is  called  its 
volume. 

Measurement  of  Extension. — A  body  has  three  di- 
mensions: length,  breadth,  and  thickness.  To  meas- 
ure these,  some  standard  is  required.  The  standard 
of  length  popularly  in  use  in  England  and  the  United 
States  is  the  yard.  Its  length  is  the  distance  between 
two  lines  on  a  certain  bar  of  bronze,  kept  in  London 
and  measured  at  a  certain  temperature,  62°F.  (see 
p.  248).  There  is  only  one  yard  in  the  world ;  all 
that  we  call  yards  are  imperfect  copies  from  it.  The 
yard  is  inconveniently  divided  into  three  feet,  or 
thirty-six  inches.  The  standard  of  length  used  in 
France,  and  by  scientific  men  throughout  the  world, 
is  the  meter.*  Its  length  is  nearly,  but  not  exactly, 

*  The  meter  is  divided  into  ten  decimeters  (dm.) ;  each  of  theae  into  ten 
centimeters  (cm.) ;  and  each  of  these  into  ten  millimeters  (mm.).  In  Kg.  1  is 


GENERAL     PROPERTIES     OF     MATTER. 


There 


PlQ.  1. 


4  o-o  0*0  ootf  °f  an  entire  meridian  of  the  earth. 
is  only  one  meter  in  the  world.  It 
is  the  length  of  a  certain  bar  of 
platinum,  kept  in  Paris,  and  meas- 
ured at  the  temperature  of  melting 
ice.  Most  copies  of  the  meter  and 
yard  are  accurate  enough  for  the 
purposes  to  which  they  are  applied. 

2.   Impenetrability   is   the    prop- 

erty of  so  occupying  space  as  to 
exclude  all  other  matter.*  JSTo  two 
bodies  can  occupy  the  same  space 
at  the  same  time.  A  book  lies  upon 
the  table  before  me  ;  no  human 
power  is  able  to  place  another  in 
the  same  spot,  until  the  first  book 


shown  a  line,  AS,  whose  length  is  a  decimeter, 
divided  into  centimeters  and  millimeters.  At  the 
side  of  it  is  another  line,  AC,  slightly  longer.  It 
is  made  up  of  four  inches,  divided  into  halves, 
quarters,  and  eighths.  The  length  of  the  meter 
is  about  39.37  inches,  or  nearly  1.1  yard. 

For  the  measurement  of  surface,  we  use  square 
meters  (sq.  m.),  square  centimeters  (sq.  cm.),  etc. 

The  unit  adopted  for  the  measurement  of  vol- 
ume is  the  cubic  decimeter.  It  is  called  a  liter. 
A  vessel  that  contains  just  a  liter  of  water  will 
hold  a  little  more  than  a  quart  of  the  same  liquid. 
Since  the  liter  has  a  length,  breadth,  and  thickness 
of  one  decimeter,  it  contains  1,000  cubic  centi- 
meters. 

*  In  common  language,  we  say  a  needle  pene- 

trates cloth,  a  nail  enters  wood,  etc.  ;  but  a  moment's  examination  shows 
that  they  merely  push  aside  the  fibers  of  the  cloth  or  wood,  and  so  press 
them  closer  together.  With  care  we  can  drop  a  quarter  of  a  pound  of 
shingle-nails  into  a  tumbler  brimful  of  water,  without  causing  it  to  over- 
flow. The  surface  of  the  water,  however,  becomes  convex. 


Comparison  of  Metric 
and  English  Measures 
of  Length. 


8  INTRODUCTION. 

is  removed.  I  attempt  to  fill  a  bottle  through  a 
closely-fitting  funnel ;  but  before  the  liquid  can  run 
in,  the  air  must  gurgle  out,  or  the  water  will  trickle 
down  the  outside  of  the  bottle. 

In  addition  to  these  two  essential  properties  of 
matter,  there  are  others  which  have  been  found  to 
be  general,  such  as  divisibility,  porosity,  and  inde- 
structibility. 

3.  Divisibility  is  that  property  by  which  a  body 
may  be  separated  into  parts.      The  extent  to  which 
the  divisibility  of  matter  may  be   carried  is  almost 
incredible.* — Example:   A  grain   of   strychnine   will 
flavor  1,750,000    grains  of  water;  hence   there  will 
be   in  each  grain  of    the    liquid  only   l  7g}  000    of    a 
grain   of   strychnine,   yet  this    amount    can    be   dis- 
tinctly tasted. 

4.  Porosity  is  the  property  of  having  pores.     By 
this   is   meant   not  the  sensible  pores  to   which   we 
refer  when  in  common  language  we  speak  of  a  por- 
ous body,  as  bread,  wood,  unglazed  pottery,  a  sponge, 
etc.,  but  the  finer  or  physical  pores.     The  latter  are 
as  invisible  to  the  eye  as  the  atoms  themselves,  and 
are  caused  by  the  fact  that  the  molecules  of  which 
a  body  is  composed  are   not  in  actual  contact,  but 

*  Newton  estimated  that  the  film  of  a  soap-bubble  at  the  instant  of 
breaking  is  less  than  g-goo  <y0o  of  an  inch  thick.  Pure  water  will  acquire  the 
requisite  viscidity  for  making  bubbles  by  adding  only  ^  part  of  soap.  It 
is  evident  that  there  must  be  at  least  one  molecule  of  soap  in  every  cubic 
jreJ-STO  of  an  inch  of  the  film,  and  that  the  molecule  must  be  smaller  than 
one  hundredth  of  a  cubic  ^-mjirres  of  an  incb-'  *•  *•»  than  *n5-s*tnnso  of  a 
cubic  inch.  Now  a  molecule  of  soft-soap  (if  it  is  a  pure  potassium  stearate, 
"Chemistry,"  p.  207)  contains  56  atoms,  and  this  point  must  be  reached 
before  we  come  to^the  possible  limit  of  divisibility. 


GENERAL     PROPERTIES     OF     MATTER.  9 

are  separated  by  minute  spaces.* — Examples:  To  a 
bowl-full  of  water  it  is  easy  to  add  a  quantity  of 
fine  salt  without  the  liquid  running  over.  Only  care 
must  be  taken  to  drop  in  the  salt  slowly,  giving  time 
for  it  to  dissolve  and  the  bubbles  of  air  to  pass  off. 
When  the  water  has  dissolved  -all  the  salt  it  can,  we 
can  still  add  other  soluble  solids,  f — In  testing  large 
cannon  by  hydrostatic  pressure  (p.  104),  water  is 
forced  into  the  gun  until  it  oozes  through  the  thick 
metal  and  covers  the  outside  of  the  gun  like  froth, 
then  gathers  in  drops  and  runs  to  the  ground  in 
minute  streams.J 

It  is  in  virtue  of  these  physical  pores  that  a  body 
changes  in  volume  when  warmed  or  cooled.  The 
molecules  become  farther  apart  or  nearer  as  heat  is 


*  These  spaces  are  so  small  that  they  can  not  be  discerned  with  the 
most  powerful  microscope,  yet  it  is  thought  that  they  may  be  very  large 
when  compared  with  the  size  of  the  atoms  themselves.  If  we  imagine  a 
being  small  enough  to  live  on  one  of  the  atoms  near  the  center  of  a  stone, 
as  we  live  on  the  earth,  then  we  are  to  suppose  that  he  might  possibly  see 
the  nearest  atoms  at  great  distances  from  him,  as  we  see  the  moon  and 
stars,  and  might  perchance  have  need  of  a  fairy  telescope  to  examine  them, 
as  we  investigate  the  heavenly  bodies.  It  is  impossible,  however,  for  us  to 
have  any  definite  knowledge  on  such  a  topic. 

t  In  this  case  we  suppose  that  the  particles  of  salt  are  smaller  than 
those  of  water,  and  those  of  the  different  substances  used  are  smaller  than 
those  of  salt.  The  particles  of  salt  fill  the  spaces  between  the  particles  of 
water,  and  the  others  occupy  the  still  smaller  spaces  left  between  the  par- 
ticles of  salt.  We  may  better  understand  this  if  we  suppose  a  bowl  filled 
with  oranges.  It  will  hold  a  quantity  of  peas,  then  of  gravel,  then  of  fine 
sand,  and  lastly  some  water. 

J  In  the  course  of  some  experiments  performed  during  the  eighteenth 
century  at  the  Florence  Academy,  Italy,  hollow  globes  of  silver  were  filled 
with  water  and  placed  in  a  screw-press.  The  spheres  being  flattened,  their 
size  was  diminished,  and  the  water  oozed  through  the  pores  of  the  metal. 
The  philosophers  of  the  day  thought  this  to  show  that  water  is  incom- 
pressible. We  now  see  that  it  proved  only  that  silver  has  pores  larger  than 
the  molecules  of  water. 


10  INTRODUCTION.  *;;   . 

applied  or  withdrawn.      We  can  not  conceive  this  to 
be  possible  if  they  are  in  perfect  contact. 

5.  Indestructibility  is  the  property  which  renders 
matter  incapable  of  being  destroyed.  We  can  not 
conceive  of  the  annihilation  of  matter.  We  may 
change  its  form,  but  we  can  not  deprive  it  of  exist- 
ence.— Example:  We  cut  down  a  tree,  saw  it  into 
boards,  and  build  a  house.  The  house  burns,  and 
only  little  heaps  of  ashes  remain.  Yet  in  the  ashes, 
*  and  in  the  smoke  of  the  burning  building,  exist  the 
identical  atoms,  which  have  passed  through  these 
various  forms  unchanged.* 


III.    SPECIFIC   PROPERTIES  OF   MATTER. 

AMONG  the  most  important  specific  properties  of 
matter  are  ductility,  malleability,  tenacity,  elasticity, 
hardness,  and  brittleness. 

1.  Ductility. — A  ductile  body  is  one  which  can  be 
drawn  into  wire.  Fig.  2  represents  a  machine  for 
making  wire.  B  is  a  steel  drawing-plate  pierced  with 
a  series  of  gradually  diminishing  holes.  A  rod  of 
iron,  A,  is  hammered  at  the  end  so  as  to  pass 

*  Walter  Raleigh,  while  smoking  in  the  presence  of  Queen  Elizabeth, 
offered  to  bet  her  majesty  that  he  could  tell  the  weight  of  the  smoke  that 
curled  upward  from  his  pipe.  The  wager  was  accepted.  Raleigh  quietly 
finished,  and  then  weighing  the  ashes,  subtracted  this  amount  from  the 
weight  of  the  tobacco  he  had  placed  in  the  pipe,  thus  finding  the  weight 
of  the  smoke.  When  we  reach  the  subject  of  combustion  in  chemistry,  we 
shall  be  able  to  detect  Raleigh's  mistake.  The  smoke  and  the  ashes  really 
weighed  more  than  the  original  tobacco,  since  the  oxygen  of  the  air  had 
combined  with  the  tobacco  in  burning. 


SPECIFIC     PROPERTIES     OF     MATTER.  11 

through  the  largest.  It  is  then  grasped  by  a  pair  of 
pincers,  (7,  and,  by  turning  the  crank,  _D,  is  drawn 
through  the  plate, 
diminished  in  di- 
ameter and  propor- 
tionately increased 
in  length.  The 
tenacity  of  the 
metal  is  greatly 

Drawing  Wire. 

improved  by  the 

process  of  drawing,  so  that  a  cable  of  fine  wire  is 
stronger  than  a  chain  or  bar  of  the  same  weight. 
Gold,  silver,  and  platinum  are  the  most  ductile 
metals.  A  silver  rod  an  inch  thick,  covered  with 
gold  leaf,  may  be  drawn  to  the  fineness  of  a  hair 
and  yet  retain  a  perfect  coating  of  gold,  three  ounces 
of  the  latter  metal  making  100  miles  of  the  gilt- 
thread  used  in  embroidery.  Platinum  wire  has  been 
drawn  so  fine  that,  though  it  is  nearly  three  times  as 
heavy  as  iron,  a  mile's  length  weighed  only  a  grain. 

2.  Malleability. — A  malleable  body  is  one  which 
can  be  hammered  or  rolled  into  sheets. — Example: 
Gold  may  be  beaten  until  it  is  only  ?60Vog-  of  an  inch 
thick.  It  would  require  1,800  such  leaves  to  equal 
the  thickness  of  common  printing-paper.*  '  Copper  is 


*  An  ingot  of  gold  is  passed  many  times  between  steel  rollers,  which 
are  so  adjusted  as  to  be  continually  brought  nearer  together.  The  metal  is 
thus  reduced  to  a  ribbon  about  ses  of  an  inch  thick.  This  is  cut  into 
inch  squares,  150  of  which  are  piled  up  alternately  with  leaves  of  strong 
paper  four  inches  square.  A  workman  with  a  hammer  beats  the  pile 
until  the  gold  is  spread  to  the  size  of  the  leaves.  Each  piece  is  next 
quartered,  and  the  600  squares  are  placed  between  leaves  of  goldbeaters' 


12 


INTRODUCTION. 


Pro.  3. 


so  malleable,  that  a  workman  can  hammer  out  a 
kettle  from  a  solid  block. 

3.  Tenacity. — A  tenacious  body  is  one  which  can 
not  easily  be  pulled  apart.    Iron  possesses  this  quality 
in  a  remarkable  degree.    Steel  wire  will  sustain  many 
thousand  times  its  own  weight. 

4.  Elasticity  is  of  four  kinds,  according  as  a  body 
tends  to  resume  its  original  form  when  compressed, 
extended,  twisted,  or  bent. 

(1.)  ELASTICITY  OF  COMPRESSION. — Many  solids,  as 
iron,  glass,  and  caoutchouc,  are  highly  elastic. — Ex- 
ample :  Spread  a  thin  coat 
of  oil  on  a  smooth  marble 
slab.  If  an  ivory  ball  be 
dropped  upon  it,  the  size 
of  the  impression  will  vary 
with  the  distance  at  which 
the  ball  is  held  above  the 
table.  This  shows  that  the 
ivory  is  flattened,  some- 
what like  a  soap-bubble 
when  it  strikes  a  smooth 

:r,,:,,,nm,CTMWa.Ml^T,,m,,,,,|;|l,l,l,      surface  and  rebounds. 

Elasticity  of  ivory.  Liquids  are  compressed 

with  great  difficulty,  so  that  for  a  long  time  they 
were  considered  incompressible.  When  the  force  is 

skin  and  pounded.  They  are  then  taken  out,  spread  by  the  breath,  cut, 
and  the  2,400  squares  pounded  as  before.  They  are  finally  trimmed  and 
placed  between  tissue-paper  in  little  books,  each  of  which  contains  twenty- 
five  gold  leaves. 


SPECIFIC     PROPERTIES     OF     MATTER.  13 

removed,  they  regain   their   exact  volume,  and  are 
therefore  perfectly  elastic. 

Gases  are  easily  compressed,  and  are  also  perfectly 
elastic.  A  pressure  of  15  Ibs.  to  the  square  inch  re- 
duces the  volume  of  water  only  -jnrfoir,  whereas  it 
diminishes  the  volume  of  a  perfect  gas  £.  A  gas  may 
be  kept  compressed  for  years,  but  on  being  released 
will  instantly  return  to  its  original  form. 

(2.)  ELASTICITY  OF  EXPANSION  is  possessed  largely 
by  very  many  substances. — Examples:  India  rubber, 
when  stretched,  tends  to  fly  back  to  its  original  di- 
mensions. A  drop  of  water  hanging  to  the  nozzle 
of  a  bottle  may  be  touched  by  a  piece  of  glass  and 
drawn  out  to  considerable  length,  but  when  let  go 
it  will  resume  its  spherical  form. 

(3.)  ELASTICITY  OF  TOESION  is  the  tendency  of  a 
thread  or  wire  which  has  been  Pto  4 

twisted,  to  untwist  again.  If  a 
weight  be  suspended  by  means  of  a 
steel  wire,  twisted  around  and  then 
released,  it  constitutes  a  torsion  pen- 
dulum. (Fig.  4.) 

(4.)  ELASTICITY  OF  FLEXURE  is  the- 
property  ordinarily  meant  by  the 
term  elastic.  Many  solids  possess 
this  quality,  within  certain  limits, 

,~  n     ,  ,  The  Torsion  Pendulum. 

to  a  high  degree.    Swords  have  been 
made  which    could   be    bent   into   a   circle    without 
breaking.      "Watch-springs,   bows,   cushions,   etc.,   are 
useful   because    of    their    elasticity.      Glass,    though 
brittle,  is  one  of  the  most  elastic  substances  known. 


14  INTRODUCTION, 

5.  Hardness. — One  body  is  harder  than  another 
when  it  will  scratch  or  indent  it.    This  property  does 
not  depend  on  density.*  —  Examples:   Gold  is  about 
2-J  times  as   dense   as  iron,  yet  it  is  much  softer. — 
Mercury  is  a  liquid,  yet  it  is  almost  twice  as  dense 
as  steel. — The  diamond   is   the  hardest  known   sub- 
stance, yet  it  is  not  one  third  as  heavy  .as  lead. 

6.  Brittleness. — A  brittle  body  is  one  that  is  easily 
broken.     This  property  is  a   frequent  characteristic 
of  hard  bodies. — Example:  Glass  will  scratch  pure 
iron,  yet  it  is  extremely  brittle. 


SUMMARY. 

MATTER  is  that  which  occupies  space.  A  separate  portion  is 
called  a  body,  and  a  particular  kind  a  substance.  A  general 
property  of  matter  belongs  to  all  substances,  and  a  specific  one 
to  particular  kinds.  Matter  is  composed  of  very  minute  atoms. 
A  group  of  atoms  forms  a  molecule,  in  which  reside  the  specific 
properties  of  a  substance.  A  physical  change  never  affects  the 
molecule,  but  a  chemical  change  breaks  it  up,  and  so  makes 
new  combinations  possible.  Physics  deals  with  physical  forces 
and  changes ;  Chemistry  with  chemical  attraction,  and  chemical 
changes.  Extension  and  impenetrability  are  the  essential  prop- 
erties of  matter.  Extension  is  the  property  of  occupying  space. 
The  amount  of  space  a  body  fills  is  its  volume.  In  virtue  of 
impenetrability  two  bodies  can  not  occupy  the  same  space  at 
the  same  time.  The  divisibility  of  matter  is  without  perceptible 
limit  so  far  as  we  know.  Porosity  is  the  property  in  virtue  of 
which  the  molecules  of  a  body  are  not  in  absolute  contact.  In- 
destructibility prohibits  the  extinction  of  matter  by  man.  Duc- 

*  A  dense  body  has  its  molecules  closely  compacted.  The  word  rare,  the 
opposite  of  dense,  is  applied  to  gases.  Mass,  or  the  quantity  of  matter  a 
body  contains,  should  be  distinguished  from  weight  or  size. 


HISTORICAL     SKETCH.  15 

tility,  malleability,  tenacity,  elasticity,  hardness,  and  brittleness 
are  the  principal  specific  properties  of  matter.  A  ductile  body 
can  be  drawn  into  wire ;  gold,  silver,  and  platinum  are  the 
most  noted  for  this  property.  A  malleable  body  can  be  ham- 
mered into  sheets ;  gold  possesses  this  quality  in  a  remarkable 
degree.  A  tenacious  body  resists  pulling  apart ;  iron  is  the  best 
example.  An  elastic  body  permits  a  play  of  its  particles,  so 
that  they  return  to  their  original  position  when  the  disturbing 
force  is  removed.  A  hard  body  can  not  easily  be  indented.  A 
brittle  body  is  readily  broken. 


HISTORICAL     SKETCH. 

IN  ancient  times,  any  seeker  after  truth  was  termed  a 
philosopher  (a  lover  of.  wisdom),  and  philosophy  included  all  in- 
vestigations concerning  both  mind  and  matter.  In  the  fourth 
century  B.C.,  Plato  assumed  4;hat  there  are  two  principles, 
matter  and  form,  which  by  combining  produce  the  five  ele- 
ments, earth,  air,  fire,  water,  and  ether.  Aristotle,  his  pupil, 
established  the  first  philosophical  ideas  concerning  matter  and 
space.  But  the  method  of  study  generally  pursued  for  2,000 
years  was  one  of  pure  metaphysical  speculation.  Observation 
had  no  place,  but  the  philosophers  made  up  a  theory,  and  then 
accommodated  facts  to  it.  They  guessed  about  the  real  essence 
of  things,  as  to  whether  matter  exists  except  when  perceived 
by  the  mind,*  and  how  a  change  in  matter  can  produce  a 
change  in  mind.  In  1620,  Bacon  published  his  "Novum  Or- 
ganum,"  advocating  the  inductive  method  of  studying  nature. 
He  argued  that  the  philosopher  should  seek  to  benefit  mankind, 
and  that,  instead  of  wasting  his  time  in  sterile  and  ingenious 
theories  about  the  world  and  matter,  he  should  watch  the 
phenomena  of  life,  gather  facts,  and  then  reasoning  from 
effects  back  to  their  causes,  reach  the  general  law.  This  work 

*  Dr.  Johnson  once  remarked  to  a  gentleman  who  had  been  defending 
the  theory  that  there  is  no  external  world,  as  he  was  going  away,  "  Pray, 
sir,  don't  leave  us,  for  we  may  perhaps  forget  to  think  of  you,  and  then 
you  will  cease  to  exist.1' 


16  INTBODUCTION. 

is  commonly  said  to  have  established  the  modern  method  of  in- 
vestigation. Ptolemy,  Archimedes,  Galileo,  and  other  physicists, 
however,  had  long  before  proved  its  value. 

The  Atomic  Theory  was  propounded  by  Democritus,  in  the 
fifth  century  B.C.,  and  twenty-two  centuries  later  elaborated  by 
Dalton,  an  English  physicist.  The  grander  generalization  and 
development  of  this  law  was  advanced  in  1811  by  Avogadro, 
an  Italian,  and  afterward  extended  by  the  French  philosopher, 
Ampere.  The  latter  asserted  that  "  equal  volumes  of  all  sub- 
stances, when  in  the  gaseous  form  and  under  like  conditions, 
contain  the  same  number  of  molecules."  For  half  a  century 
this  hypothesis  was  ignored.  Its  adoption  within  the  past  thirty 
years  produced  fundamental  changes  in  chemistry.  Displacing 
the  old  it  gave  birth  to  a  new  and  consistent  notation,  and,  raised 
to  the  rank  of  a  law,  it  served  as  a  criterion  by  which  the  correct- 
ness of  its  formulae  might  be  tested. 

The  history  of  the  establishment  of  a  standard  of  measures 
is  a  curious  one.  Anciently,  length  was  referred  to  some  por- 
tion of  the  human  body,  as  the  foot ;  the  cubit  (the  length  of 
the  fore-arm  from  the  elbow  to  the  end  of  the  middle  finger) ; 
the  finger's  length  or  breadth ;  the  hand's  breadth ;  the  span, 
etc.  In  England,  Henry  I.  (1120)  ordered  that  the  ell,  the  an- 
cient yard,  should  be  the  exact  length  of  his  arm.  Afterward 
a  standard  yard-stick  was  kept  at  the  Exchequer  in  London ; 
but  it  was  so  inaccurate,  that  a  commissioner,  who  examined  it 
in  1742,  wrote:  "A  kitchen  poker  filed  at  both  ends  would 
make  as  good  a  standard.  It  has  been  broken,  and  then  re- 
paired so  clumsily  that  the  joint  is  nearly  as  loose  as  a  pair  of 
tongs."  In  1760,  Mr.  Bird  carefully  prepared  a  copy  of  this  for 
the  use  of  the  government.  It  was  not  legally  adopted  until 
1824,  when  it  was  ordered  that  if  destroyed,  it  should  be  re- 
stored by  a  comparison  with  the  length  of  a  pendulum  vibrating 
seconds  at  the  latitude  of  London.  At  the  great  fire  in  London, 
1834,  the  Parliament  House  was  burned,  and  with  it  Bird's 
yard-stick.  Repeated  attempts  were  then  made  to  find  the 
length  of  the  lost  standard  by  means  of  the  pendulum.  This 
was  found  impracticable,  on  account  of  errors  in  the  original 
directions.  At  last  the  British  government  adopted  a  standard 
prepared  from  the  most  reliable  copies  of  Bird's  yard-stick.  A 


HISTORICAL     SKETCH.  17 

copy  of  this  was  taken  by  Troughton,  an  instrument-maker  of 
London,  for  the  use  of  our  Coast  Survey.* 

The  French  had  previously  adopted  for  the  length  of  the 
legal  foot  that  of  the  royal  foot  of  Louis  XIV.,  as  perishable  a 
standard  as  Henry's  arm.  In  1790,  the  Prince  de  Talleyrand 
proposed  to  the  Constituent  Assembly  of  France  the  foundation 
of  a  system  based  on  a  single  and  universal  standard,  which 
might  be  used  by  all  civilized  nations.  The  selection  of  this 
was  committed  to  five  members  of  the  Academy  of  Sciences, 
MM.  Borda,  Lagrange,  Laplace,  Monge,  and  Condorcet,  who 
decided  that  the  ten-millionth  part  of  a  quarter  of  the  earth's 
meridian  should  be  taken  as  the  standard  of  length,  from  which 
the  standards  of  surface,  volume,  capacity,  and  weight  should 
be  derived.  A  trigonometric  measurement  was  made  of  the 
arc  of  a  meridian  extending  through  France  from  Dunkirk  to 
Barcelona,  a  work  which  occupied  seven  years.  In  1799,  an 
international  commission  was  assembled  at  Paris,  with  repre- 
sentatives from  most  of  the  governments  of  Europe.  They 
deposited  at  the  Palace  of  the  Archives,  in  Paris,  the  standard 
meter-bar  of  platinum,  and  the  standard  kilogram  weight,  made 
of  the  same  metal.  In  English  denominations  the  length  of  the 
meter  is  almost  exactly  3.28  feet,  or  39.37  inches ;  the  weight 
of  the  kilogram  almost  exactly  2.2  pounds  avoirdupois. 

But  after  the  establishment  of  the  metric  system  it  was 
found  that  a  slight  mistake  had  been  made  in  the  measurement 
of  the  arc  of  the  meridian.  The  English,  who  had  declined  to 
accept  the  French  system,  discovered  also  a  difficulty  in  the 
determination  of  the  yard  from  the  pendulum  beating  seconds. 
Both  the  yard  and  the  meter  are  therefore  arbitrary  and  not 
absolute  standards.  Copies  of  each  have  been  made  so  care- 
fully and  distributed  so  widely  that  there  is  no  probability  of 
any  appreciable  loss  resulting  from  the  accidental  destruction 
of  the  originals.  The  metric  system  is  by  far  the  simplest  and 
best  in  use,  but  it  has  not  yet  generally  supplanted  other  sys- 

*  A  bronze  bar.  which,  has  the  standard  length  at  61.79°  P.,  has  been 
presented  by  the  English  government  to  that  of  the  United  States.  Ac- 
cording to  Act  of  Congress,  sets  of  weights  and  measures  have  been  dis- 
tributed to  the  governors  of  the  several  States.  Both  the  yard  and  the 
meter  are  legal  standards  in  the  United  States  and  Great  Britain. 


18  INTRODUCTION. 

terns  that  retain  their  popularity,  not  on  account  of  merit,  but 
only  because  of  human  conservatism  and  the  inconveniences 
resulting  from  change. 

Consult  Cooke's  "New  Chemistry,"  chapter  on  Molecules,  etc.; 
Powell's  "History  of  Natural  Philosophy";  Buckley's  "History 
of  Natural  Science";  Whewell's  "History  of  the  Inductive 
Sciences";  Roscoe's  "John  Dalton  and  his  Atomic  Theory,"  in 
Manchester  Science  Lectures,  '73-4;  "Appleton's  Cyclopedia," 
Art.  Molecules;  Outer  bridge's  "Divisibility  of  Gold  and  Other 
Metals,"  in  Popular  Science  Monthly,  Vol.  XL,  p.  74;  Crookes' 
"  The  Radiometer — a  fresh  evidence  of  a  Molecular  Universe," 
Popular  Science  Monthly,  Vol.  XIII. ,  p.  1;  Tait's  "Recent  Ad- 
vances in  Physical  Science,"  Chap.  XII.,  The  Structure  of  Mat- 
ter ;  Hoefer,  "  Histoire  de  la  Physique  et  de  la  Chimie";  Draper's 
"History  of  Intellectual  Development  of  Europe";  Barnard  on 
"The  Metric  System." 


II. 

MOTION  AND  FORCE. 


BEST  is  nowhere.  The  winds  that  come  and  go,  the  ocean  that  un- 
easily throbs  along  the  shore,  the  earth  that  revolves  about  the  sun,  the 
light  that  darts  through  space— all  tell  of  a  universal  law  of  Nature.  The 
solidest  body  hides  within  it  inconceivable  velocities.  Even  the  molecules 
of  granite  and  iron  have  their  orbits  as  do  the  stars,  and  move  as  cease- 
lessly. 

No  energy  is  ever  lost.  It  changes  its  form,  but  the  eye  of  philosophy 
detects  it  and  enables  us  to  drive  it  from  its  hiding-place  undiminished. 
It  assumes  Protean  guises,  but  is  every-where  a  unit.  It  may  disappear 
from  the  earth;  still— 

"Somewhere  yet  that  atom's  force 
Moves  the  light-poised  universe." 


ANALYSIS  OF  MOTION  AND  FORCE. 


1.  DEFINITIONS. 

2.  COMMUNICATION  OF  MOTION. 

3.  RESISTANCES  TO  MOTION. 


MOTION   AND 
FORCE. 


4.   FIRST  LAW  OF  MOTION 


(1.)  Inertia. 


(2.)  Momentum. 

5.  SECOND  LAW  OF  MOTION. 

6.  THIRD  LAW  OF  MOTION. 

7.  COMPOSITION  OF  MOTIONS. 

8.  COMPOSITION  OF  FORCES. 

9.  TRIANGLE  OF  FORCES. 

10.  POLYGON  OF  FORCES. 

11.  RESOLUTION  OF  FORCES. 

12.  MOTION  IN  A  CURVE. 

13.  CIRCULAR  MOTION. 

14.  REFLECTED  MOTION. 

15.  ENERGY. 

16.  KINETIC  AND  POTENTIAL  ENERGY. 

17.  CONSERVATION  OF  ENERGY, 


MOTION  AND  FORCE. 

1.  Motion  is  change  of  place.  All  motion,  as 
well  as  rest,  with  which  we  are  acquainted,  is  rela- 
tive.— Examples :  When  we  ride  in  the  cars,  we  judge 
of  our  motion  by  the  objects  around  us. — A  man  on 
a  steamer  may  be  in  motion  with  regard  to  the 
shore,  but  at  rest  with  reference  to  the  objects  on 
the  deck  of  the  vessel.  Force  is  that  which  produces 
or  tends  to  produce  or  to  destroy  motion.  Velocity  is 
the  rate  at  which  a  body  moves.  When  the  rate  is 
constant,  i.  e.,  when  the  moving  body  passes  over  equal 
spaces  in  equal  times,  the  body  moves  with  constant 
velocity.  It  is  expressed  by  the  number  of  units  of 
space  through  which  the  body  moves  in  a  unit  of  time. 
— Example:  Ten  miles  an  hour  or  fifteen  feet  a  sec- 
ond. When  the  rate  is  variable,  i.  e.,  when  the  spaces 
passed  over  by  the  body  in  equal  times  are  unequal, 
the  body  moves  with  variable  velocity. — Example: 
The  velocity  of  a  train  between  two  stations  is  vari- 
able. In  cases  of  this  kind  it  is  convenient  to  speak 
of  the  average  velocity  of  the  train,  which  is  the  ratio 
of  the  whole  distance  passed  over  to  the  time  required 
for  such  passage.  The  train  will  evidently  arrive  at 
the  second  station  in  the  same  time,  whether  it  move 
uniformly  with  the  average  velocity  or  with  its  ac- 
tually varying  velocity. 


22  MOTION     AND     FORCE. 

2.  The  Communication  of  Motion  is  not  instan- 
taneous.   If  I  strike  one  end  of  a  rail  a  mile  long  the 
tremor  communicated  from  particle  to  particle  will 
take  a  definite  time  to  reach  the  other  end.    A  stone 
thrown  against   a  pane   of  glass   shatters  it,  but  a 
bullet  fired  through  it  will  make  only  a  round  hole. 
The  bullet  is  gone  before  the  motion  has  i^me  to  be 
communicated  to  the  surrounding  particles.    A  frac- 
tion of  time  is  required  for  a  ball  to  receive  the  force 
of  the  exploding  powder  and  get  under  full  headway. 

3.  The  Resistances  to  Motion  are  friction  and  the 
resistance  of  air  and  water.    (1.)  Friction  is  the  resis- 
tance caused  by  the  surface  over  which  a  body  moves. 
It  is  of  great  value  in  common  life.    Without  it,  nails, 
screws,  and  strings  would  be  useless ;   engines  could 
not  draw  the  cars ;  we  could  hold  nothing  in  our  hands ; 
and  we  should  everywhere  walk  as  on  glassy  ice.    (2.) 
The  resistance  which  a  body  meets  in  passing  through 
air  or  water  is  caused  largely  by  the  particles  displaced. 

LAWS  OF  MOTION. — There  are  three  laws  of  mo- 
tion, which  were  first  distinctly  formulated  by  Sir 
Isaac  Newton. 

4.  First  Law  of  Motion. — A  body  set  in  motion 
will  move  forever  in  a  straight  line  with  constant 
velocity,  unless  acted  on  by  some  external  force.    Ob- 
viously, no  experiment  will  directly  prove  this  law. 
Common  experience,  moreover,  seems  to  contradict 
it ;  for  everywhere  on  the  surface  of  the  earth  bodies 
in  motion  show  a  tendency  to  stop.     A  little  reflection 
will  show,  however,  that  these  bodies  are  subject  to 


LAWS     OF     MOTION.  23 

the  action  of  external  forces  tending  to  arrest  the 
motion.  It  is  supposed  that  could  these  resistances 
be  removed  a  body  once  in  motion  would  never  stop. 
Inertia. — The  law  just  stated  is  often  called  the 
law  of  inertia.  Matter  has  no  inherent  power  of 
producing  change  upon  itself.  If  a  body  be  already 
in  motion,  force  has  to  be  expended  in  stopping  it. 
If  it  be  at  rest,  force  is  required  to  start  it  in  mo- 
tion. In  either  case  we  "  overcome  its  inertia."  The 
danger  in  jumping  from  a  car  in  rapid  motion  lies 
in  the  fact  that  the  body  has  the  speed  of  the  train, 
while  the  forward  motion  of  the  feet  is  checked  by 
contact  with  the  ground.  It  is  necessary  to  jump  as 
nearly  as  possible  in  the  direction  in  which  the  train 
is  moving,  and  be  ready  to  run  the  instant  the  feet 
touch  the  ground.  Those  who  do  so  can  then  grad- 
ually overcome  the  inertia  of  the  body,  and  after  a 
few  yards  can  turn  as  they  please. 

Momentum. — To  measure  any  force  we  must  know 
what  quantity  of  matter  is  moved,  and  what  velocity 
it  receives  in  a  unit  of  time.  The  quantity  of  matter 
in  a  body  is  called  its  mass.  It  is  not  the  same  as 
weight,  but  is  proportional  to  this  (see  p.  57),  so 
that  we  speak  of  pounds  of  mass  as*  well  as  pounds 
of  weight.  The  product  of  mass  by  velocity  is  called 
momentum*  Thus  if  a  mass  of  five  pounds  move 

*  A  heavy  body  may  be  moving  very  slowly  and  yet  have  an  immense 
momentum. — Examples ;  An  iceberg,  with  a  scarcely  perceptible  motion,  will 
crush  the  strongest  ship  as  if  it  were  an  egg-shell.— Soldiers  have  thought 
to  stop  a  spent  cannon-ball  by  putting  a  foot  against  it,  but  have  found  its 
momentum  sufficient  to  break  a  leg. 

On  the  other  hand,  a  light  body  moving  with  a  high  velocity  may  have 


24  MOTION    AND     FORCE. 

with  a  velocity  of  twenty  feet  per  second,  it  has  one 
hundred  units  of  momentum. 

5.  Second  Law  of  Motion. — A  force  acting  upon 
a  ~body  in  motion  or  at  rest,  produces  the  same  effect 
whether  it  acts  alone  or  with  other  forces. — Examples : 
All  bodies  upon  the  earth  are  in  constant  motion 
with  it,  yet  we  act  with  the  same  ease  that  we 
should  were  the  earth  at  rest.* — We  throw  a  stone 

an  enormous  momentum.  —  Examples :  The  air  in  a  hurricane  will  tear  up 
trees  by  the  roots  and  level  buildings  to  the  ground.— Sand  driven  from  a 
tube  by  steam  is  used  for  drilling  and  in  stone-cutting,  engraving,  etc. 

"In  a  rude  age,  before  the  invention  of  means  for  overcoming  friction, 
the  weight  of  bodies  formed  the  chief  obstacle  to  setting  them  in  motion. 
It  was  only  after  some  progress  had  been  made  in  the  art  of  throwing 
missiles,  and  in  the  use  of  wheel-carriages  and  floating  vessels,  that  men's 
minds  became  practically  impressed  with  the  idea  of  mass  as  distinguished 
from  weight.  Accordingly,  while  almost  all  the  metaphysicians  who  dis- 
cussed the  qualities  of  matter,  assigned  a  prominent  place  to  weight  among 
the  primary  qualities,  few  or  none  of  them  perceived  that  the  sole  unalter- 
able property  of  matter  is  its  mass.  At  the  revival  of  science  this  property 
was  expressed  by  the  phrase  '  The  inertia  of  matter ' ;  but  while  the  men 
of  science  understood  by  this  term  the  tendency  of  the  body  to  persevere 
in  its  state  of  motion  (or  rest),  and  considered  it  a  measurable  quantity, 
those  philosophers  who  were  unacquainted  with  science  understood  inertia 
in  its  literal  sense  as  a  quality — mere  want  of  activity  or  laziness  I  there- 
fore recommend  to  the  student  that  he  should  impress  his  mind  with  the 
idea  of  mass  by  a  few  experiments,  such  as  setting  in  motion  a  grindstone 
or  a  well-balanced  wheel,  and  then  endeavoring  to  stop  it,  twirling  a  long 
pole,  etc.,  till  he  comes  to  associate  a  set  of  acts  and  sensations  with  the 
scientific  doctrines  of  dynamics,  and  he  will  never  afterward  be  in  any  dan- 
ger of  loose  ideas  on  these  subjects."— MAXWELL'S  "Theory  of  Heat,"  p.  85 

*  A  ball  thrown  up  into  the  air  with  a  force  that  would  cause  it  to  rise 
fifty  feet,  will  ascend  to  that  height  whatever  horizontal  wind  may  be  blow- 
ing.—While  riding  on  a  car,  we  throw  a  stone  at  some  object  at  rest.  The 
stone,  having  the  motion  of  the  train,  strikes  just  as  far  ahead  of  the 
object  as  it  would  have  gone  had  it  remained  on  the  train.  In  order  to 
hit  the  mark,  we  should  have  aimed  a  little  back  of  it.— The  circus-rider 
wishes,  while  his  horse  is  at  full  speed,  to  jump  through  a  hoop  suspended 
before  him.  He  simply  springs  directly  upward.  Going  forward  by  the 
momentum  which  he  had  acquired  before  he  leaped  from  the  horse,  he 
passes  through  the  hoop  and  alights  upon  the  saddle  again.— A  person 


LAWS     OF     MOTION. 


25 


directly  at  an  object  and  hit  it,  yet,  within  the  sec- 
ond, the  mark  has  gone  forward  many  feet.* — If  a 
cannon-ball  be  thrown  horizontally,  it  will  fall  as  fast 
and  strike  the  earth  as  soon  as  if  dropped  to  the 
ground  from  the  muzzle  of  the  gun.  In  Fig.  5,  D  is 
an  arm  driven  by  a  wooden  spring,  E,  and  turning 

Fio.  5. 


Illustration  of  the  Second  Law  of  Motion. 

on  a  hinge  at  C.  At  D  is  a  hollow  containing  a 
bullet,  so  placed  that  when  the  arm  is  sprung,  the 
ball  will  be  thrown  in  the  line  FK.  At  F  is  a  simi- 
lar ball,  supported  by  a  thin  slat,  G,  and  so  arranged 
that  the  same  blow  which  throws  the  ball,  D,  will 
let  the  ball,  F,  fall  in  the  line  FH.  The  two  balls 
will  strike  the  floor  at  the  same  instant. 

6.  Third  Law  of  Motion. — Action  is  equal  to  re- 
action, and  in  the  contrary  direction. — Examples:  A 
bird  in  flying  beats  the  air  downward,  but  the  air 

riding  in  a  coach  drops  a  cent  to  the  floor.  It  apparently  strikes  where  it 
would  if  the  coach  were  at  rest. 

*  The  earth  moves  in  its  orbit  around  the  sun  at  the  rate  of  about 
eighteen  miles  ner  second.  (See  "Fourteen  Weeks  in  Astronomy,"  p.  106.) 


26 


MOTION     AND     FORCE. 


FIG.  6. 


reacts  and  supports  the  bird. — The  powder  in  a  gun 
explodes  with  equal  force  in  every  direction,  driving 
the  gun  backward  and  the  ball  forward,  with  the 
same  momentum.  Their  velocities  vary  with  their 
weights ;  the  heavier  the  gun,  the  less  will  the  recoil 
be  noticed. — When  we  spring  from  a  boat,  unless  we 
are  cautious,  the  reaction  will  drive  it  from  the 
shore. — When  we  jump  from  the  ground,  we  tend  to 
push  the  earth  from  us,  while  it  reacts  and  pushes 
us  from  it ;  we  separate  from  each  other  with  equal 

momentum,  and  our  veloc- 
ity is  as  much  greater 
than  that  of  the  earth  as 
we  are  lighter. — We  walk 
therefore  by  reason  of  the 
reaction  of  the  ground  on 
which  we  tread. 

The  apparatus  shown  in 
Fig.  6  consists  of  ivory 
balls  hung  so  as  to  vibrate  readily.*  If  a  ball  be  let 
fall  from  one  side,  it  will  strike  the  second  ball, 
which  will  react  with  an  equal  force,  and  stop  the 
motion  of  the  first,  but  transmit  the  motion  to  the 
third ;  this  will  act  in  the  same  manner,  and  so  on 
through  the  series,  each  acting  and  reacting  until 
the  last  ball  is  reached ;  this  will  react  and  then 
bound  off,  rising  as  high  as  the  first  ball  fell  (except 
the  loss  caused  by  resistances  to  motion).  If  two 

*  The  same  experiments  can  be  performed  by  means  of  glass  marbles 
or  billiard  balls  placed  in  a  groove.  Better  still,  attach  strings  to  glass 
marbles  by  means  of  mucilage  and  bits  of  paper  and  suspend  them  from  a 
simple  wooden  frame. 


Illustration  of  the  Third  Law  of  Motion. 


COMPOSITION     OF     FORCES.  27 

balls  be  raised,  two  will  fly  off  at  the  opposite  end; 
if  two  be  let  fall  from  one  side  and  one  from  the 
other,  they  will  respond  alternately. 

7.  Composition    of  Motions.  —  Let   a   ball    at   A 
(Fig.  7)  be  acted  on  by  a  force  which  would  drive 
it  in  a  given  time  to  B,  and  also  at  the  same  instant 
by  another  which  would  drive  it  to  D  in  the  same 
time;  the  ball  will  move  in  the  direction  AC. — Ex- 
amples :  A  person  wishes  to  row  a                ^  ^ 
boat  across  a  swift  current  which     A^ B 

v^^  y 

would    carry   him    down    stream.  \^^^^      \ 

He  therefore  steers  toward  a  point  \___^X 

above   that   which    he    wishes   to  Com£8ition  of  MotionsC 
reach,  and  so  goes  directly  across. 
—A  bird,  beating  the  air  with  both  its  wings,  flies 

in  a  direction  different  from  that  which  would  be 
given  by  either  one. 

8.  Composition  of  Forces.— When  a  body  is  thus 

acted  on  by  two  forces, 
we  draw  lines  represent- 
ing their  directions,  and 
mark  off  AD  and  AB, 
whose  lengths  represent 
their  comparative  mag- 
nitudes. We  next  com- 
plete the  parallelogram 
and  draw  the  diagonal 
AC,  which  denotes  the 
composition  of  TWO  Forces.  resultant  of  these  forces, 

and  gives  the  direction  in  which  the  body  will  move. 


28 


MOTION     AND     FORCE. 


If  more  than  two  forces  act,  we  find  the  resultant 
of  two,  then  of  that  resultant  and  a  third  force,  and 
so  on. 

9.  Triangle  of  Forces. — In  Fig.  8  the  resultant, 
AC,  could  have  been  obtained  more  easily  by  draw- 
ing AB  to  represent  the  magnitude  and  direction 
of  one  force,  and  then  similarly  BO  for  the  other 
force.  Connecting  the  initial  point,  A,  of  the  first 
line  with  the  terminal  point,  C,  of  the  second  line, 
we  have  AC  for  the  magnitude  and  direction  of  the 
resultant,  which  completes  a  triangle. 

1C.  Polygon  of  Forces.— Let  Ft,  Fz,  Fz,  F±,  and 

F5  (Fig.  9),  represent  five  forces  acting 
on  the  same  point  at  the  same  time. 
To  find  their  resultant,  we  draw  (Fig. 
10)  OA,  AB,  BO,  OD,  and  DE,  equal 
and  parallel  respectively  to  F^,  F^,  F3, 
Ft,  and  F6.  Then,  joining  the  first  point 
with  the  last,  we  have 
Five  Forces  acting  OE  to  represent  the 

at  the  same  Point. 


Pie.  9. 


Polygon  of  Forces. 


tion  of  their  combined  resultant. 
For  OB  is  the  resultant  for  OA  and 
AB;  OC  for  OB  and  BC ;  OD  for 
OC  and  CD ;  and  OE  for  OD  and 
DE.  This  method  is  applicable  to 
the  representation  of  any  number  of  forces. 

11.  Resolution  of  Forces  consists  in  finding  what 
forces  are  equivalent  to  a  given  force  under  special 
conditions.  A  triangle  is  drawn,  having  the  given 


RESOLUTION     OF     FORCES. 


29 


force  as  one  side. — Example :  There  is  a  wind,  blow- 
ing nearly  from  the 
west  (Fig.  11)  against 
the  sail,  AS,  of  a  vessel 
going  northward.  We 
may  regard  the  wind- 
force,  WC,  as  the  re- 
sultant of  two  forces, 
WD  and  DC.  The 
former,  being  parallel 
to  the  sail,  is  not  effect- 
ive ;  the  latter  is  per- 
pendicular to  it,  and 
tends  to  drive  the  ves- 

-i-iji  Eesolution  of  Forces.    Ship  sailing  northward. 

sel  nearly  north-east. 

Again,  resolving  D<7,  we  find   this  equivalent  to  two 

forces,  DE  and  EG.  The 
former  pushes  the  vessel 
sideways,  but  is  largely 
counteracted  by  the  re- 
sistance of  the  water 
against  the  broad  side ; 
EC  is  in  the  direction 
of  the  ship's  course,  and 
propels  it  north. 

By  shifting  the  rig- 
ging, one  vessel  may 
sail  into  the  harbor 
while  another  is  sailing 
out,  both  driven  by  the 
same  wind.  In  Fig.  12, 


FIG.  12. 


Resolution  of  Forces.    Ship  sailing  south- 
ward. 


80  MOTION     AND     FORCE. 

which  represents  a  ship  sailing  southward,  the  letter- 
ing and  explanation  is  the  same  as  for  Fig.  11,  if 
we  substitute  "south"  for  "north."*  If  the  ship 
were  required  to  go  westward,  it  would  tack  alter- 
nately NW  and  SW.  In  this  way  its  resultant  direc- 
tion might  be  almost  in  the  "teeth  of  the  wind." 

A  canal-boat  drawn  by  horses  is  acted  upon  by  a 
force  which  tends  to  bring  it  to  the  bank.  This 
force  may  be  resolved  into  two,  one  pulling  toward 
the  tow-path,  and  the  other  directly  ahead.  The 
former  is  counteracted  by  the  shape  of  the  boat  and 
the  action  of  the  rudder ;  the  latter  draws  the  boat 
forward. 

12.  Motion  in  a  Curve. — To  change  the  direction 
of  a  moving  body  requires  an  external    force  (p.  22, 
§  4).     When  this  force  is  a  continuous  force  and  the 
direction  in  which  it  acts  differs  continually  from  the 
path  in  which  the  body  moves,  the  body  will  describe 
a  curved  line. — Example:   A  body  thrown  obliquely 
into  the  air  describes  a  curve,  because  the  force  of 
gravity  continually  changes  the  direction  of  the  mov- 
ing  body. 

13.  Circular  Motion  is  produced  when  a  moving 
body  is  drawn  toward  a  center  by  a  constant  force. 

*  In  a  similar  manner  we  may  resolve  the  three  forces  which  act  upon 
a  kite— viz.,  the  pull  of  the  string,  the  force  of  the  wind,  and  its  own 
weight.  In  Fig.  11,  let  AB  represent  the  face  of  the  kite.  We  can  resolve  W C, 
the  force  of  the  wind,  into  WD  and  DC.  We  next  resolve  DC  into  DE  and 
EC.  We  then  have  a  force,  EC,  which  overcomes  the  weight  of  the  kite 
and  tends  to  lift  it  upward.  The  string  pulls  in  the  .direction  CD,  perpen- 
dicularly to  the  face.  The  kite  obeys  neither  one  of  these  forces  alone,  but 
both,  and  so  ascends  in  a  direction  CA  between  the  two.  It  is  really  drawn 
up  an  inclined  plane  by  the  joint  force  of  the  wind  and  the  string. 


CIRCULAR     MOTION.  31 

Thus,  when  a  sling  is  whirled,  the  stone  is  pulled 
toward  the  hand  by  the  string,  and  as,  according  to 
the  third  law  of  motion,  every  action  has  its  equal 
and  opposite  reaction,  the  hand  is  pulled  toward  the 
stone.  If  the  string  break,  the  stone  will  continue 
to  move,  according  to  the  first  law  of  motion,  in  a 
straight  line  in  the  direction  of  a  tangent  to  the 
circle  at  that  point.  The  tension  of  the  string,  act- 
ing inward,  is  called  the  Centripetal  (centrum,  the 
center,  petere,  to  seek)  force ;  and  the  reaction  of 
the  stone  upon  the  string,  acting  outward,  is  termed 
the  Centrifugal  (centrum,  the  center,  fugere,  to  flee) 
force.* 

The  following  examples  are  among  those  usually 

*  It  should  be  noticed  that  in  circular  motion  there  is  but  one  true 
force  concerned.  It  acts,  however,  upon  a  body  in  motion.  The  so-called 
centrifugal  force  has  nothing  to  do  with  the  production  of  the  motion, 
being  merely  the  resistance  which  the  body  offers  by  its  inertia  to  the 
operation  of  the  centripetal  force,  and  ceases  the  instant  that  force  is  dis- 
continued. It  does  not  act  at  right  angles  to  the  centripetal  force,  as  is 
often  stated,  but  in  direct  opposition.  A  body  never  flies  off  from  the 
center  impelled  by  the  centrifugal  force,  since  that  can  never  exceed  the 
centripetal  (action  =  reaction),  and  moreover  the  path  of  such  a  body  is  in 
the  direction  of  a  tangent,  and  not  the  radius  of  a  circle.  Thus,  when 
water  is  thrown  off  a  grindstone  in  rapid  rotation,  the  tendency  of  the 
water  to  continue  to  move  on  in  the  direction  of  the  straight  line  in  which 
it  is  going  at  each  instant  (in  other  words,  the  inertia  of  the  water)  over- 
comes its  adhesion  to  the  stone,  and  it  flies  off  in  obedience  to  the  first  law 
of  motion.  So,  also,  when  a  grindstone,  driven  at  a  high  speed,  breaks, 
and  the  fragments  are  thrown  with  great  velocity,  we  are  not  to  suppose 
that  the  centrifugal  force  impels  them  through  the  air.  That  force  existed 
only  while  the  stone  was  entire.  It  was  opposed  to  the  force  of  cohesion, 
and  in  the  moment  of  its  triumph  ceased,  and  the  fragments  of  the  stone 
fly  off  in  virtue  of  the  velocity  they  possess  at  that  instant.  Again,  the 
so-caUed  centrifugal  force  is  not  a  real  force  urging  bodies  upward  at  the 
equator.  The  earth's  surface  is  merely  falling  away  from  a  tangent,  and, a 
part  of  the  force  of  gravity  is  spent  in  overcoming  the  inertia  of  bodies. 
The  term  centrifugal  force  has  caused  much  confusion,  and  will  perhaps 
be  discarded. 


32 


MOTION     AND     FORCE. 


given  to  illustrate  the  action  of  the  center-fleeing 
force :  Water  flies  from  a  grindstone  on  account  of 
the  centrifugal  force  produced  in  the  rapid  revolu- 
tion, which  overcomes  the  adhesion. — In  factories, 
grindstones  are  sometimes  revolved  with  such  veloc- 
ity that  this  force  overcomes  that  of  cohesion,  and 
the  ponderous  stones  fly  into  fragments. — A  pail  full 

of  water  may  be  whirled 
around  so  rapidly  that 
none  will  spill  out,  be- 
cause the  centrifugal 
force  overcomes  that  of 
gravity. — When  a  horse  is 
running  around  a  small 
circle,  he  bends  inward 
to  overcome  the  centrif- 
ugal force. 

The  heavenly  bodies 
present  the  grandest  ex- 
ample of  circular  motion.  We  may  suppose  the  earth 
to  have  been  moving  originally  in  the  direction  AE. 
The  attraction  of  the  sun,  however,  drawing  it  in 
the  direction  US,  it  passes  along  the  line  EE'.  If 
the  centripetal  force  were  suddenly  to  cease,  the  earth 
would  fly  off  into  space  along  a  tangent,  as  EA.  The 
rapid  revolution  of  the  earth  on  its  axis  tends  to 
throw  off  all  bodies  headlong.  As  this  acts  in  oppo- 
sition to  gravity,  it  diminishes  the  weight  of  bodies 
at  the  equator,  where  it  is  greatest,  being  there 
equivalent  to  -^  of  the  force  of  gravity.  It  also 
tends  to  drive  the  water  on  the  earth  from  the  poles 


Circular  Motion. 


REFLECTED     MOTION; 


33 


FIG.  14. 


toward  the  equator.  Were  the  velocity  of  the  earth's 
rotation  to  diminish,  the  water 
would  flow  back  toward  the  poles, 
and  tend  to  restore  the  earth  to 
a  spherical  form.*  This  influence 
is  well  illustrated  by  the  appara- 
tus shown  in  Fig.  14.  The  hoop 
is  made  to  slide  upon  its  axis, 
and  if  revolved  rapidly  it  will 
assume  an  oval  form,  bulging  out 
more  and  more  as  the  velocity  is 
increased.f 

14.  Reflected  Motion  is  produced  by  the  reaction 
of  a  surface  against  which  an  elastic  body  is  cast. 
If  a  perfectly  elastic  ball  be  thrown  in  the  direction 

*  Since  the  earth's  polar  diameter  is  nearly  twenty-seven  miles  shorter 
than  its  equatorial  diameter,  we  are  not  sure  that  this  motion  of  its  waters 
would  make  it  perfectly  spherical. 

t  This  apparatus  is  accompanied  by  objects  to  illustrate  the  principle 
that  all  bodies  tend  to  revolve  about  their  shortest  diameters.  "  Tie  to  the 
middle  of  a  lead-pencil  a  piece  of  string  about  three  feet  long.  Suspend  so 
that  the  pencil  will  balance  itself.  Now  twist  the  end  of  the  string  between 
the  thumb  and  the  first  finger  of  the  right  hand,  steadying  and  holding 
the  string  with  the  left  hand.  A  circular  motion  will  thus  be  communi- 
cated to  the  pencil,  and  it  will  revolve  around  the  point  on  which  it  is 
suspended.  Tie  a  piece  of  white  string  around  the  middle  of  the  pencil, 
or  its  center  of  gravity,  simply  to  show  the  position  of  that  point.  Now 
tie  the  first  piece  of  string  half-way  between  the  end  of  the  pencil  and 
the  center  of  gravity,  and  communicate  the  circular  motion  described 
above,  and  we  shall  observe  that  the  pencil  will  still  revolve  around  the 
center  of  gravity,  the  point  marked  by  the  white  string  being  at  rest.  It 
can  thus  be  shown  that  any  thing,  of  whatever  shape,  will  tend  to  revolve 
on  its  shortest  diameter.  If  the  end  links  of  a  small  steel  chain  (such  as 
is  often  attached  to  purses  or  parasols)  be  hooked  together,  the  string  tied 
to  a  link,  and  the  circular  motion  given,  it  will  be  observed  that  the  chain 
begins  to  take  an  elliptical  form,  which  gradually  approaches  that  of  a 
circle,  until  at  last  it  becomes  a  circle,  when  it  revolves  horizontally. 
This  shows  that  even  a  ring  revolves  on  its  shortest  axis." 


34  MOTION     AND     FOKCE. 

OP  against  the  surface  AS,  it  will  rebound  in  the 
line  PQ.  The  angle,  i,  between  the  direction  OP  and 
the  perpendicular,  PR,  drawn  at  the  point  of  inci- 
dence, is  called  the  angle  of  incidence.  The  angle 

of  reflection,  r,  is 
that  between  this 
perpendicular,  PR, 
and  the  direction 
PQ.  If  OP  repre- 
sent  the  magnitude 

Reflected  Motion.  and  directi(m  of  the 

incident  force,  it  may  be  resolved  into  OR  and  RP. 
But  the  reaction,  PR,  is  equal  to  the  vertical  portion, 
RP,  of  the  incident  force,  while  the  horizontal  por- 
tion is  not  checked.  Hence  PQ  =  OP,  and  the  angle 
of  incidence  is  equal  to  the  angle  of  reflection. 

15.  Energy  has  been  defined  as  the  power  of  produc- 
ing change  of  any  kind  (p.  4,  §  5).  To  produce  a  change 
resistance  must  be  overcome,  and  "  work  is  done  when 
resistance  is  overcome."  Therefore  energy  may  also  be 
defined  as  the  power  of  doing  work.  It  is  in  general 
a  power  put  into  a  body  by  means  of  work,  and  which 
comes  out  of  it  when  it  does  work. — Examples:  A  wound- 
up clock,  a  red-hot  iron.  The  difference  between  energy 
and  momentum  is  easily  illustrated.  When  a  bullet  is 
fired  from  a  rifle,  the  momenta  of  both  are  equal,  but 
the  energy  of  the  former,  i.  e.,  its  power  of  doing  work, 
as  piercing  a  board,  is  far  greater.  Energy  is  propor- 
tional to  the  square  of  the  velocity  of  the  moving  body. 
Thus,  a  cannon-ball  given  double  speed  will  penetrate 
four  times  as  far  into  a  wall ;  and  a  stone  thrown  up- 


ENEKG-Y.  35 

ward  at  the  rate  of  ninety- six  feet  per  second  will  rise 
nine  times  as  far  as  with  a  velocity  of  thirty-two  feet. 

1G.  Two  Forms  of  Energy. — Energy  may  be  either 
active  or  latent.  When  a  rock  is  tumbling  down  a  moun- 
tain-side, it  exhibits  the  force  of  gravity  in  full  sway ; 
but  when  the  rock  was  lodged  on  the  mountain-top,  it 
possessed  the  same  energy,  which  could  be  developed 
at  any  moment  by  loosening  it  from  its  place.  These 
two  forms  are  known  as  energy  of  motion  and  energy 
of  position,  or  kinetic  and  potential  energy.* 

17.   Conservation  of  Energy. — The  sum  of  all  the 

energy  in  the  universe  remains  the  same  while  its 
transformations  are  infinite.  One  kind  of  energy  is 
changed  into  another ;  from  an  available  form  to  one 
that  is  not  controllable.  A  hammer  falls  by  the  force 
of  gravity.  In  coming  to  rest  when  stopped,  it  does 
the  work  of  crushing  what  it  hits,  and  its  motion  as 
a  mass  is  converted  into  one  of  molecules,  revealing 

*  The  following  may  be  taken  as  examples  to  show  the  difference 
between  kinetic  and  potential  energy.  We  wind  a  watch,  and  by  a  few 
moments  of  labor  condense  in  the  spring  a  potential  energy,  which  is  doled 
out  for  twenty-four  hours  in  the  kinetic  energy  of  the  moving  wheels  and 
hands.  Lift  a  pendulum,  and  you  thereby  give  the  weight  potential  energy. 
Let  it  fall,  and  the  potential  changes  gradually  to  kinetic.  At  the  center 
of  the  arc  the  potential  is  gone  and  kinetic  is  possessed.  Then  the  kinetic 
changes  again  to  potential,  which  increases  till  the«nd  of  the  arc  is  reached 
and  the  pendulum  ceases  to  rise,  when  the  energy  is  that  of  position,  not 
of  motion.  Potential  energy  is  like  what  is  concealed,  lying  in  wait  and 
ready  to  burst  forth  on  the  instant.  It  is  that  of  a  loaded  gun  prepared 
for  the  arm  of  the  marksman.  It  is  that  of  a  river  trembling  on  the  brink 
of  a  precipice,  about  to  take  the  fearful  leap.  It  is  that  of  a  weight  wound 
up  and  held  against  the  tug  of  gravity.  It  is  that  of  the  engine  on  the 
track  with  the  steam  hissing  from  every  crevice.  On  the  contrary,  kinetic 
energy  is  that  in  actual  operation.  The  bullet  is  speeding  to  the  mark; 
the  river  is  tumbling ;  the  weight  is  falling ;  the  engine  is  flying  over  the 
rails.  It  is  that  of  heat  radiating  from  our  fires,  and  electricity  carrying  our 
messages  over  the  continent. 


36  MOTION     AND     FORCE. 

itself  to  our  touch  as  heat.  The  sun  is  continually 
sending  forth  radiant  energy,  which  has  been  stored 
up  in  it  by  the  aggregation  of  matter  during  untold 
ages.  Its  kinetic  energy  is  thus  becoming  dissipated 
into  potential  energy;  but,  even  after  it  ceases  to 
glow,  the  grand  total,  including  all  that  was  once 
kinetic,  will  remain  unchanged. 


PRACTICAL     QUESTIONS. 

1.  A  rifle-ball  thrown  against  a  board  standing  edgewise,  will  knock  it 
down ;  the  same  bullet  fired  at  the  board  will  pass  through  it  without  dis- 
turbing its  position.    Why  is  this? 

2.  Why  can  a  boy  skate  safely  over  a  piece  of  thin  ice,  when,  if  he 
should  pause,  it  would  break  under  him  directly? 

3.  Why  can  a  cannon-ball  be  fired  through  a  door  standing  ajar,  with- 
out moving  it  on  its  hinges? 

4.  Why  can  we  drive  on  the  head  of  a  hammer  by  simply  striking  the 
end  of  the  handle? 

5.  Suppose  you  were  on  a  train  of  cars  moving  at  the  rate  of  30  miles 
per  hour;  with  what  velocity  would  you  be  thrown  forward  if  the  train 
were  stopped  instantly? 

6.  In  what  line  does  a  stone  fall  from  the  mast-head  of  a  vessel  in 
motion  ? 

7.  If  a  ball  be  dropped  from  a  high  tower,  it  will  strike  the  ground  a 
little  east  of  a  vertical  line.    Why  is  this? 

8.  It  is  stated  that  a  suit  was  once  brought  by  the  driver  of  a  light 
wagon  against  the  owner  of  a  coach  for  damages  caused  by  a  collision. 
The  complaint  was  "the  latter  was  driving  so  fast  that  when  the  two  car- 
riages struck,  the  driver  of  the  former  was  thrown  forward  over  the  dash- 
board."   On  trial  he  was  nonsuited,  because  his  own  evidence  showed  him 
to  be  the  one  who  was  driving  at  the  unusual  speed.    Explain. 

9.  Suppose  a  train  moving  at  the  rate  of  30  miles  per  hour:  on  the 
rear  platform  is  a  spring  gun  aimed  parallel  to  the  track  and  in  a  direction 
precisely  opposite  to  the  motion  of  the  car.     Let  a  ball  be  discharged  with 
the  exact  speed  of  the  train;  where  would  it  fall? 

10.  Suppose  a  steamer  in  rapid  motion,  and  on  its  deck  a  man  jumping. 
Can  he  jump  farther  by  leaping  the  way  the  boat  is  moving  than  in  the 
opposite  direction? 

11.  Could  a  party  play  ball  on  the  deck  of  an  ocean  steam-ship  when 


PRACTICAL     QUESTIONS.  37 

steaming  along  at  the  rate  of  20  miles  per  hour,  without  making  allowance 
for  the  motion  of  the  ship  ? 

12.  Since  action  "is  equal   to  reaction,  why  is  it  not  so  dangerous  to 
receive  the  '•  kick "  of  a  gun  as  the  force  of  the  bullet? 

13.  If  you  were  to  jump  from  a  carriage  in  rapid  motion,  would  you 
leap  directly  toward  the  spot  on  which  you  wished  to  alight? 

14.  If  you  wished  to  shoot  a  bird  in  swift  flight,  would  you  aim  di- 
rectly at  it? 

15.  At  what  parts  of  the  earth  is  the  centrifugal  force  least  ? 

16.  What  causes  the  mud  to  fly  from  the  wheels  of  a  carriage  in  rapid 
motion  ? 

17.  What  proof  have  we  that  the  earth  was  once  a  soft  mass? 

18.  On  a   curve  in  a  railroad,  one  track  is  always  higher  than  the 
other.    Why  is  this? 

19.  What  is  the  principle  of  the  sling? 

20.  The  mouth  of  the  Mississippi  River  is  about  2J  miles  farther  from 
the  center  of  the  earth  than  its  source.     In  this  sense  it  may  be  said  to 
"run  up  hill."     What  causes  this  apparent  opposition  to  the  attraction  of 
gravity  ? 

21.  Is  it  action  or  reaction  that  breaks  an  egg,  when  I  strike  it  against 
the  table? 

22.  Was  the  man  philosophical  who  said  that  it  "was  not  the  falling 
so  far,  but  the  stopping  so  quick,  that  hurt  him "  ? 

23.  If  one  person  runs  against  another,  which  receives  the  greater 
blow? 

24.  Would  it  vary  the  effect  if  the  two  persons  were  running  in  oppo- 
site directions?     In  the  same  direction? 

25.  Why  can  you  not  fire  a  rifle-ball  around  a  hill? 

26.  Why  is  it  that  a  heavy  rifle  "kicks"  less  than  a  light  shot-gun? 

27.  A  man  on  the  deck  of  a  large  vessel  draws  a  small  boat  toward 
him.    Can  you  express  the  ratio  of  the  ship's  motion  to  that  of  the  boat? 

28.  Suppose  a  string,  fastened  at  one  end,  will  just  support  a  weight  of 
25  Ibs.  at  the  other.    Unfasten  it,  and  let  two  persons  pull  upon  it  in  oppo- 
site directions.    How  much  can  each  pull  without  breaking  it? 

29.  Can  a  man  standing  on  a  platform-scale  make  himself  lighter  by 
lifting  up  on  himself  ? 

30.  Why  can  not  a  man  lift  himself  by  pulling  up  on  his  boot-straps? 

31.  With  what  momentum  would  a  steam-boat  weighing  1,000  tons,  and 
moving  with  a  velocity  of  10  ft.  per  second,  strike  against  a  sunken  rock? 

32.  With  what  momentum  would  a  train  of  cars  weighing  100  tons, 
and  running  10  miles  per  hour,  strike  against  an  obstacle? 

33.  What  would  be  the  comparative  kinetic  energy  of  two  hammers, 
one  driven  with  a  velocity  of  20  ft.  per  second  and  the  other  10  ft.  ? 

34.  If  a  100  horse-power  engine  can  propel  a  steamer  5  miles  per  hour, 
will  one  of  200  horse-power  double  its  speed  ? 

35.  Why  are  ships  becalmed  at  sea  often  floated  by  strong  currents 
into  dangerous  localities  without  the  knowledge  of  the  crew? 


38  MOTION     AND     FORCE. 

36.  A  man  in  a  wagon  holds  a  50-lb.  weight  in  his  hand.     Suddenly 
the  wagon  falls  over  a  precipice.     Will  he,  while  dropping,  bear  the  strain 
of  the  weight? 

37.  Why  are  we  not  sensible  of  the  rapid  motion  of  the  earth? 

38.  A  feather  is  dropped  from   a   balloon  which  is  immersed  in  and 
swept  along  by  a  swift  current  of  air.     Will  the  feather  be  blown  away  or 
will  it  appear  to  drop  directly  down? 

39.  Suppose  a  bomb-shell,  flying  through  the  air  at  the  rate  of  500  ft. 
per  second,  explodes  into  two  parts  of  equal  weight,  driving  one  half  for- 
ward in  the  same  direction  as  before,  but  with  double  its  former  velocity. 
What  would  become  of  the  other  half? 

40.  Which  would  have  the  greater  penetrating  power,  a  10-lb.  cannon- 
ball  with  a  velocity  of  1,000  ft.  per  second,  or  a  100-lb.  ball  with  a  velocity 
of  100  ft.  per  second? 

41.  There  is  a  story  told  of  a  man  who  erected  a  huge  pair  of  bellows 
in  the  stern  of  his  pleasure-boat,  that  he  might  always  have  a  fair  wind. 
On  trial,  the  plan  failed.     In  which  direction  should  he  have  turned  the 
beUows? 

42.  If  a  man  and  a  boy  were  riding  in  a  wagon,  and,  on  coming  to  the 
foot  of  a  hill,  the  man  should  take  up  the  boy  in  his  arms,  would  that 
help  the  horse? 

43.  If  we  whirl  a  pail  of  water  swiftly  around  with  our  hands,  why 
will  the  water  tend  to  leave  the  center  of  the  pail? 

44.  Why  will  the  foam  collect  at  the  hollow  in  the  center? 

45.  If  two  cannon-balls,  one  weighing  8  Ibs.  and  the  other  2  Ibs.,  be 
fired  with  the  same  velocity,  which  will  go  the  farther? 

46.  Resolve  the  force  of  the  wind  which  turns  a  common  windmill, 
and  show  how  one  part  acts  to  push  the  wheel  against  its  support,  and  one 
to  turn  it  around. 

47.  When  an  animal  is  jumping  or  falling,  can  any  exertion  made  in 
mid-air  change  the  motion  of  its  cent^r  of  gravity? 

48.  If  one  is  riding  rapidly,  in  which  direction  will  he  be  thrown  when 
the  horse  is  suddenly  stopped? 

49.  When  standing  in  a  boat,  why,  as  its  starts,  are  we  thrown  back- 
ward? 

50.  When  carrying  a  cup  of  tea,  if  we  move  or  stop  quickly,  why  is 
the  liquid  liable  to  spill  ? 

51.  Why,  when  closely  pursued,  can  we  escape  by  dodging? 

52.  Why  is  a  carriage  or  sleigh,  when  sharply  turning  a  corner,  liable 
to  tip  over? 

53.  Why,  if  you  place  a  card  on  your  finger  and  on  top  of  it  a  cent,  can 
you  snap  the  card  from  under  the  cent,  which  will  then  drop  on  your  finger? 

54.  Why  is  a  "running  jump"  longer  than  a  "standing  jump"? 

55.  Why,  after  the  sails  of  a  vessel  are  furled,  does  it  still  continue  to 
move  ?  and  why,  after  the  sails  are  spread,  does  it  require  some  time  to  get 
it  tinder  full  headway  ? 

56.  Why  can  a  tallow  candle  be  fired  through  a  board  ? 


SUMMAKY.  39 


SUMMARY. 

MATTER,  so  far  as  we  know  it,  is  in  constant  change. 
Change  of  place  is  termed  motion.  Terrestrial  motion  is  re- 
stricted by  friction,  by  the  air,  and  by  water.  Friction  is 
caused  by  the  roughness  of  the  surface  over  which  a  body 
moves.  It  may  be  decreased  by  the  use  of  grease  to  fill  up  the 
minute  projections,  or  by  changing  the  sliding  into  rolling  fric- 
tion. Air  and  water  must  be  displaced  by  a  moving  body ;  the 
resistance  they  offer  is  measured  by  the  kinetic  energy  expended 
in  overcoming  it,  and  is  hence  proportional  to  the  square  of  its 
velocity.  Motion  takes  place  in  accordance  with  three  laws ; 
viz. :  A  moving  body  left  to  itself  tends  to  go  forever  in  a 
straight  line  with  a  constant  velocity ;  a  force  has  the  same  effect 
whether  it  acts  alone  or  with  other  forces,  and  upon  a  body  at 
rest  or  in  motion ;  and  action  is  equal  and  opposed  to  reaction. 
By  means  of  the  principles  of  the  composition  and  resolution  of 
forces,  we  can  find  the  individual  effect  of  a  single  force  or  the 
combined  effect  of  several  forces.  To  change  the  path  of  a  mov- 
ing body  requires  an  external  force.  When  the  direction  of  the 
action  of  this  force  continually  differs  from  the  direction  of  the 
motion  of  the  body,  the  path  of  the  latter  is  that  of  a  curved  line. 
A  particular  case  of  this  is  circular  motion.  The  path  of  a 
bullet  or  rocket  in  the  air  exemplifies  curvilinear  motion;  and 
the  movement  of  a  stone  whirled  in  a  sling  and  of  a  planet  re- 
volving about  the  sun  are  illustrations  of  circular  motion.  When 
a  rubber  ball  bounds  back  from  a  surface  against  which  it  is 
thrown,  the  angle  of  reflection  equals  the  angle  of  incidence. 

Energy  is  the  power  of  overcoming  resistance,  i.e.,  of  doing 
work.  This  power,  possessed  by  bodies,  may  be  either  latent — 
potential— as  in  the  case  of  a  suspended  weight,  or  actual- 
kinetic — as  in  the  case  of  the  weight  when  falling.* 

*  When  energy  refers  to  that  possessed  by  visible  masses  it  is  called  molar 
energy ;  when  it  refers  to  the  energy  possessed  by  molecules  it  is  called  molecu- 
lar energy.  A  body  is  hot  because  of  the  kinetic  energy  of  its  molecules. 
Heat  is,  therefore,  an  example  of  molecular  energy.  Light,  sound,  electricity 
are  other  forms  of  molecular  energy.  The  different  forms  of  energy  are 
mutually  convertible.  The  molar  energy  of  a  falling  mass  when  stopped  is 
transferred  to  its  molecules  and  becomes  the  molecular  energy  of  heat.  This 


40  MOTIOK     AND     FORCE. 


HISTORIC  AL      SKETCH. 

ARISTOTLE  taught  that  all  motion  is  naturally  circular,  and 
this  view  was  held  by  his  school.  He  divided  the  phenomena 
of  motion  into  two  classes — the  natural  and  the  violent.  As  an 
instance  of  the  former,  he  gave  the  falling  of  a  stone,  which 
constantly  increases  in  velocity ;  and  of  the  latter,  a  stone 
thrown  vertically  up,  which  being  against  nature,  continually 
goes  slower.  Newton,  in  his  "Principia,"  published  in  1687, 
propounded  the  laws  of  motion  as  now  received.  Other  philos- 
ophers, notably  Galileo,  Hooke,  and  Huyghens,  had  anticipated 
much  of  his  reasoning,  yet  so  slowly  were  his  opinions  accepted 
that  "at  his  death,"  says  Voltaire,  "he  had  not  more  than 
twenty  followers  outside  of  England." 

The  law  of  the  Conservation  of  Energy,  Faraday,  the  great 
English  physicist,  pronounced  "the  grandest  ever  presented  for 
the  contemplation  of  the  human  mind."  It  has  been  established 
within  the  present  century ;  yet  we  now  know  that  former 
scholars  had  inklings  of  the  wonderful  truth.  It  arose  in  con- 
nection with  discoveries  on  the  subject  of  Heat,  and  its  history 
will  be  treated  of  hereafter. 

Consult  Stewart's  "Conservation  of  Energy";  Youmans' 
"Correlation  of  the  Physical  Forces";  Faraday's  "Lectures  on 
the  Physical  Forces";  Everett's  "  Deschanel's  Natural  Philos- 
ophy"; Tait's  "Recent  Advances  in  Physical  Science";  Max- 
well's "Matter  and  Motion *' ;  "Appleton's  Cyclopedia,"  Art. 
Correlation  of  Forces;  Tyndall's  "Crystalline  and  Molecular 
Forces,"  in  Manchester  Science  Lectures,  '73-4;  Crane's  "Ball 
Paradox,"  in  Popular  Science  Monthly,  Vol.  X.,  p.  725. 

energy  in  the  steam-engine  is  reconverted  into  the  molar  energy  of  the  train. 
Every  change  that  takes  place  in  the  universe  is  a  case  either  of  transfer- 
ence or  transformation  of  energy.  Energy,  though  convertible,  is  as  inde- 
structible as  matter— it  is  conserved.  The  principle  of  the  Conservation  of 
Energy  teaches  that  the  quantity  of  energy  in  the  universe  is  constant, 
that  it  can  neither  be  increased  nor  diminished.  "We  cannot  account  for 
its  origin;  we  only  know  its  law  of  action,  which  we  must  finally  refer  to  a 
Supreme  Being. 


III. 

ATTRACTION. 


"  THE  smallest  dust  which  floats  upon  the  wind 
Bears  this  strong  impress  of  the  Eternal  mind: 
In  mystery  round  it  subtle  forces  roll, 
And  gravitation  binds  and  guides  the  whole." 

"  Attraction,  as  gravitation,  is  the  muscle  and  tendon  of  the  universe, 
by  which  its  mass  is  held  together  and  its  huge  limbs  are  wielded.  As 
cohesion  and  adhesion,  it  determines  the  multitude  of  physical  features  of 
its  different  parts.  As  chemical  or  interatomic  action,  it  is  the  final  source 
to  which  we  trace  all  material  changes."— ABNOTT. 


ANALYSIS  OF  MOLECULAR  FORCES. 


'  — ATTRACTIVE  AND  REPELLENT  FORCES. 


1.  COHESION.  * 


2.  ADHESION.  H 


II.  ATTRACTION 

OF 
GRAVITATION. 


1.  Definition  of  Cohesion. 

2.  Three  States  of  Matter. 

3.  Cohesion  acts  at  Insensible  Distances. 

4.  Surface  Tension  of  Liquids. 

5.  Liquids  Tend  to  Form  Spheres. 

6.  Solids  Tend  to  Form  Crystals. 

7.  Annealing  and  Tempering. 

8.  Rupert's  Drop. 

1.  Definition  and  Illustration  of  Adhe- 

sion. 

f  (1.)  Direct. 

2.  Capillarity.  J  (2.)  Reversed. 

[  (3.)  Law  of  Capillarity. 

3.  Imbibition. 

4.  Solution. 

5.  Diffusion  of  Liquids. 

6.  Diffusion  of  Gases. 

7.  Osmose. 

1.  Law  of  Gravitation. 

2.  Illustrations  of  Gravity. 

3.  Mass  and  Weight. 

4.  The  Earth's  Center  of  Gravity. 

5.  The  Center  of  Gravity  of  a  Body. 

6.  Laws  of  Weight. 

C  (1.)  In  a  Vacuwm. 

(2.)  In   the  Air,  with  At- 

7.  Falling  I  ^^  MacM^ 

Bodies.  I  (3)  ExpeTiments  with  this 
{_  Machine. 

8.  Equations  of  Bodies  Falling  Freely. 

9.  Measurement  of  Kinetic  Energy. 

10.  Resistance   of   the   Air   to   Falling 

Bodies. 

11.  Equilibrium. 

1 12.  The  Pendulum. 


ATTRACTION. 


I.    MOLECULAR  FORCES. 

Attractive  and  Repellant  Forces. — If  we  take  a 
piece  of  iron  and  attempt  to  pull  it  to  pieces,  we  find 
that  there  is  a  force  which  holds  the  molecules  to- 
gether and  resists  our  efforts.  If  we  try  to  compress 
the  metal,  we  find  that  there  is  a  force  which  holds 
the  molecules  apart  and  resists  our  efforts  as  before. 
We  thus  see  that  there  are  two  opposing  forces — ah 
attraction  and  a  repulsion — which  operate  between  the 
molecules  and  resist  change  of  distance  between 
them. 

1.    COHESION. 

1.  Cohesion   is  that   force  which  holds  together 
molecules  of  the  same  kind. 

2.  Three    States    of   Matter. — Matter  occurs   in 
three  states — solid,  liquid,  and   gaseous.    The   mole- 
cules of  a  solid,  are  held  together  by  cohesion,  and  this 
force  resists  increase  of  distance  between  them.    This 
resistance  may,  however,  be  overcome  by  the  applica- 
tion of  heat-energy.    The  molecules  of  a  solid  are  not 
really  fixed  and  at  rest,  but  move  swiftly  to  and  fro 
within  narrow  limits,  without  being  able  to  leave  these 
limits.    By  the  application  of  heat  the  path  over  which 


44  ATTRACTION. 

the  molecules  move  and  the  intensity  of  their  motion 
is  increased,  and  the  solid  expands.  At  a  certain  stage 
in  this  process  of  heating  the  distance  between  the  mole- 
cules becomes  great  enough  to  permit  them  to  wander 
slowly  through  the  substance.  The  solid  has  now  be- 
come a  liquid.  Matter  in  this  state  has  no  independent 
form,  but  assumes  the  shape  of  the  containing  vessel. 
Continuing  with  the  application  of  heat,  the  spaces  be- 
tween the  molecules  become  finally  so  great  that  the 
attraction  between  them  is  canceled,  permitting  them 
to  dart  hither  and  thither  with  great  velocity.  This  rep- 
resents the  gaseous  state.  In  this  state  the  molecules 
require  to  be  restrained  by  the  walls  of  the  containing 
vessel.  By  pressure  and  cooling  gases  are  liquefied ; 
by  further  cooling  liquids  are  rendered  solid. 

3.  Cohesion  Acts  at  Insensible  Distances. — Take 
two  bullets,  and  having  flattened  and  cleaned  one 
side  of  each,  press  them  together  with  a  twisting 
motion.  They  will  cohere  when  the  molecules  are 
crowded  into  apparent  contact.* — If  two  globules  of 
mercury  be  brought  near  each  other,  at  the  instant 
they  seem  to  touch  they  will  suddenly  coalesce. — Two 
freshly-cut  surfaces  of  rubber,  when  warmed  and 
pressed  together,  will  cohere  as  if  they  formed  one 
piece. — The  process  of  welding  illustrates  this  princi- 
ple. A  wrought-iron  tool  being  broken,  we  wish  to 
mend  it.  So  we  bring  the  iron  to  a  white  heat  at  the 


*  Surfaces  may  appear  to  the  eye  to  be  in  contact  when  they  are  not 
actually  so.  Newton  found,  during  some  experiments  on  light  (p.  220),  that 
a  convex  lens  or  a  watch-glass  laid  on  a  flat  glass  does  not  touch  it,  and 
can  not  be  made  to  do  so,  even  by  a  force  of  many  pounds. 


COHESION.  45 

ends  which  we  intend  to  unite.  This  partly  over- 
comes the  attraction  of  cohesion,  and  the  molecules 
will  move  easily  upon  one  another.  Laying  now  one 
of  the  two  heated  ends  upon  the  other,  we  pound 
them  until  the  molecules  are  brought  near  enough 
for  cohesion  to  grasp  them. 

4.  Surface  Tension  of  Liquids. — Within  a  liquid 
each  molecule  is  pressed  by  the  weight  of  all  those 
above  it,  so  that  the  slight  cohesion  between  it  and 
its   neighbors   is   masked.     At  the   surface   there    is 
no   such   liquid   pressure.    Cohesion   causes   the   sur- 
face film  to  be  in  a  state  of  tension  like  a  sheet  of 
stretched  India  rubber.    A  double  film  may  be  de- 
tached by  using  soapy  water  and  lifting  out  of  it  the 
bowl  of  a  pipe.    The  elasticity  and  toughness  of  the 
film   are  shown  by  blowing  it  into  a  bubble,  whose 
surface  is  many  times  greater  than  that  across  the 
bowl  that  held  it.    A  candle  flame  may  be  blown  out 
by  the  bubble  as  it  contracts  toward  its  own  center 
and  expels  a  breeze  of  air  through  the  tube  of  the 
pipe.* 

5.  Liquids  Tend  to  Form  Spheres. — Mix   alcohol 
and  water  in  such  proportion  that 'a  drop  of  olive-oil 


*  There  are  many  charming  experiments  that  can  be  made  with  soap 
films.  (See  Popular  Science  Monthly,  Vol.  IX.,  p.  575 ;  Scientific  American,  May 
15,  1886;  Scientific  American  Supplement,  Jan.  25,  1879.)  A  good  recipe  for 
making  soap  solution  is  as  follows :  Procure  some  of  the  best  white  Castile, 
or  palm-oil  soap.  Scrape  from  it  about  four  ounces  of  thin  shavings,  put 
these  into  a  quart  bottle  of  purest  rain-water,  or  distilled  water,  and  shake 
until  the  strongest  clear  solution  of  the  soap  is  had.  Then  add  a  pint  of 
pure  concentrated  glycerine.  A  film  from  this  mixture  will  last  for  hours, 
and  bubbles  over  a  foot  in  diameter  are  easily  made. 


ATTRACTION. 


will  sink  into  it  without  going  to  the  bottom.  The 
action  of  gravity  on  it  is  just  balanced  by  the  buoy- 
ancy of  the  liquid,  so  that  cohesion  can  act  without 
much  interference.  Like  a  soap-bubble,  the  outside 
film  of  the  oil  tends  to  contract  upon  its  interior, 
and  a  nearly  perfect  sphere  remains  suspended.  The 
contraction  of  the  tough  surface  film  is  what  pro- 
duces the  roundness  of  dew-drops,  rain-drops,  glob- 
ules of  mercury,  and  melted  lead  as  it  falls  and  cools 
into  round  shot. 

6.  Solids  Tend  to  Form  Crystals. —  A  liquid  in 
becoming  solid  shows  a  tendency  to  assume  a  regular 
form  bounded  by  plane  surfaces  termed  a  crystal. 

FIG.  16. 


Ice  Flowers. 

Crystals  of  different  substances  show  in  most  cases 
marked  differences  in  form,  which  may  serve  for  the 
recognition  of  the  substance.*  When  different  sub- 

*  Epsom  salt  crystallizes  in  four-sided  prisms,  common  salt  in  cubes, 
and  alum  in  octahedra.  We  can  illustrate  the  formation  of  the  last  by 
adding  alum  to  hot  water  until  no  more  will  dissolve.  Then  suspend  strings 
across  the  dish  and  set  it  away  to  cool.  Beautiful  octahedral  crystals  will 


COHESION.  47 

stances  are  contained  in  the  same  solution,  they  sep- 
arate on  crystallization,  and  each  molecule  goes  to  its 
'own.  The  exquisite  beauty  of  these  crystalline  forms 
is  seen  in  snow-flakes  and  the  frost-work  traced  on  a 
cold  morning  upon  the  windows  or  the  stone-flagging. 
A  beam  of  light  passed  through  a  block  of  ice  reveals 
these  crystals  as  a  mass  of  star-like  flowers  (Fig.  16).* 
Melted  iron  rapidly  cooled  in  a  mold  has  not  time 
to  arrange  its  crystals.  If,  however,  the  iron  be  after- 
ward violently  jarred,  as  when  used  for  cannon,  rail- 
cars,  etc.,  the  molecules  take  on  the  crystalline  form 
and  the  metal  becomes  brittle. f 

7.  Annealing   and    Tempering.  —  If   a     piece    of 
wrought-iron  be  heated  and  then  plunged  into  water, 
it  becomes  hard  and  brittle.    If,  on  the  contrary,  it 
be  heated  and  cooled   slowly,  it  is  made  tough  and 
flexible.     Steel  is  tempered  by  heating  red-hot,  then 
cooling  quickly,  and  afterward  re-heating  and  cooling 
slowly.    It  becomes  then  one  of  the  most  elastic  and 
tough  of  known  substances. 

8.  The  Rupert's  Drop  is  a  tear  of  melted  glass 
dropped  into  water,  and  cooled  quickly.    The  exte- 

collect  on  the  threads  and  sides  of  the  vessel.  The  slower  the  process,  the 
larger  the  crystals. 

*  It  is  noticeable  that,  as  the  crystals  melt,  at  the  center  of  each  liquid 
flower  Is  a  vacuum,  showing  that  there  is  not  enough  water  formed  to  fill 
the  space  occupied  by  the  crystal,  and  that  the  solid  contracts  as  it  passes 
into  a  fluid  (p.  271).  This  experiment  is  easily  tried.  The  ice  must  be  cut 
parallel  to  the  plane  of  its  freezing  and  be  not  over  half  an  inch  thick.  A 
common  oil  lamp  will  furnish  the  light. 

t  On  examining  such  a  piece  of  iron,  which  can  easily  be  procured  at 
a  car  or  machine  shop,  we  can  see  in  a  fresh  fracture  the  smooth,  shin- 
ing faces  of  the  crystals. 


48  ATTRACTION. 

rior  at  once  becomes  rigid,  while  the  interior  is  still 
hot  and  expanded.  When  the  whole  mass  is  cool,  the 
interior  is  in  a  state  of  strain.  If  the  tail 
of  the  drop  be  nipped  off,  so  that  the  ex- 
terior shell  is  broken,  the  tension  will  cause 
the  mass  to  fly  into  powder  with  a  sharp 
explosion.  All  glassware,  when  first  made, 
is  brittle,  but  it  is  annealed  by  being 

Rupert's  Drop. 

drawn  slowly  through  a  long  oven,  highly 
heated  at  one  end,  but  quite  cool  at  the  other.  Dur- 
ing this  passage,  the  molecules  of  glass  have  time 
to  arrange  themselves  in  a  stable  position.* 


PRACTICAL     QUESTIONS. 

1.  Why  can  we  not  weld  a  piece  of  copper  to  one  of  iron? 

2.  Why  is  a  bar  of  iron  stronger  than  one  of  wood  ? 

3.  Why  may  a  piece  of  iron,  when  perfectly  welded,  be  stronger  than 
before  it  was  broken  ? 

4.  Why  do  drops  of  different  liquids  vary  in  size? 

5.  When  you  drop  medicine,  why  will  the  last  few  drops  contained  in 
the  bottle  be  of  a  larger  size  than  the  others? 

6.  Why  are  the  drops  larger  if  you  drop  them  slowly? 

7.  Why,  if  you  melt  scraps  of  lead,  will  they  form  a  solid  mass  when 
cooled  ? 

8.  In  what  liquids  is  the  force  of  cohesion  greatest? 

9.  Why  does  iron,  by  continued  jarring,  become  brittle? 

10.  Why  can  glass  be  welded? 

11.  Name  some  substances  that  can  not  be  welded.     Why  not? 

12.  What  liquids  would  you  select  for  showing  surface  tension? 

*  "  The  restoration  of  cohesion  is  beautifully  seen  in  the  gilding  of  china. 
A  figure  is  drawn  upon  the  china  with  a  mixture  of  oxide  of  gold  and  an 
essential  oil.  The  article  is  then  heated,  whereby  the  essential  oil  and  the 
oxygen  of  the  gold  are  expelled,  and  a  red-brown  pattern  remains.  This 
consists  of  pure  gold  in  a  finely-divided  state,  without  luster.  By  rubbing 
with  a  hard  burnisher,  the  particles  of  gold  cohere  and  reflect  the  rich 
yellow  color  of  the  polished  metal." 


ADHESION. 


49 


2.    ADHESION. 

1.  Adhesion   is  the    force   which   holds   together 
molecules  of  different  kinds. — Examples:  Two  pieces 
of  wood  are  fastened  together  with  glue,  two  pieces 
of  china  with  cement,  two   bricks  with  mortar,  two 
sheets  of  paper  with  mucilage,  and  two  pieces  of  tin 
with  solder. — Syrup  and  coal-oil  are  purified  by  filter- 
ing through  animal  charcoal. — Bubbles  can  be  blown 
from  soap-suds,  because  the  soap  by  its  adhesive  force 
holds  together  the  particles  of  water. 

2.  Capillarity. — If  there  is  strong  adhesion  between 
a  liquid  and  a  solid  partly  immersed  in  it,  the  liquid 
rises  above  the  general  level  and  wets  the  solid,  caus- 
ing the  surface  film  along  the  line  of         PIO.  is. 
contact  to  be  concave  upward.    This  is 

shown  in  Fig.  18,  which  represents  a 
tube  of  glass  dipped  in  water.  On  the 
inner  side  of  the  tube  the  concavity  is 
more  marked  in  proportion  as  the  bore 
FIG.  19.  is  less.  The  contraction  of  the 
surface  film  (p.  45)  balances 
the  weight  of  the  liquid  above  Direct  capillarity, 
the  level  within  the  tube.  This  is  called 
capillarity  (capillus,  a  hair),  because  best 
shown  in  tubes  with  a  bore  as  fine  as  a  hair. 
If  the  tube  be  thrust  into  a  liquid,  like 
mercury,  which  does  not  wet  it,  the  capil- 
larity is  reversed ;  the  liquid  is  convex  up- 
ward, and  within  the  tube  it  is  depressed. 
A  striking  illustration  of  the  effect  of  narrowing 


Keversed 
Capillarity. 


50 


ATTRACTION. 


FIG.  20. 


The  Capillary  Curve. 


the  exposed  surface  of  the  film  may  be  seen  by 
putting  two  clean  glass  plates  edgewise  into  colored 
water '  so  that  their  lower  part 
shall  be  immersed,  with  a  pair  of 
upright  edges  touching  (Fig.  20.) 
Just  at  the  edge  the  liquid  rises 
to  the  top ;  the  height  decreases 
as  the  successive  distances  be- 
tween the  two  plates  increases, 
and  the  liquid  surface  seen  edge- 
wise forms  a  curve  called  the 
Hyperbola. 

Law  of  Capillarity.*— The  height  to  which  a  liquid 
rises  in  a  wetted  tube  varies  inversely  as  the  diam- 
eter.— Example:  In  a  tube  whose  inner  diameter  is 
1  mm.  (see  p.  12),  water  rises  at  ordinary  tempera- 
ture (62°F)  to  a  height  of  29  mm.  (a  little  over  an 
inch) ;  if  the  tube  be  only  |  mm.  in  diameter,  the 
height  will  be  twice  as  great.  Cold  water  rises  higher 
than  warm  water,  or  than  alcohol  or  ether. 

3.  Imbibition. — Many  porous  bodies  (sensible  pores, 
p.  8),  such  as  sugar,  blotting  paper,  sand,  a  lamp 
wick,  a  towel,  absorb  liquids  at  a  rate  which  is  in- 
creased if  the  materials  be  warm.t  This  is  due  to  the 

*  For  further  discussion  of  Surface  Tension  and  Capillarity,  consult 
Maxwell's  "Theory  of  Heat,"  p.  280;  "American  Journal  of  Science," 
Dec..  1882,  p.  416;  Pickering's  "Physical  Manipulation,"  Vol.  I.,  p.  102; 
Daniells'  "Physics,"  p.  245;  Deschanel's  "Natural  Philosophy,"  p.  130. 

t  In  the  same  way,  water  is  drawn  to  the  surface  of  the  ground  to  fur- 
nish vegetation  with  the  materials  of  growth.  Even  in  the  winter,  when 
the  surface  is  frozen,  the  water  still  finds  its  way  upward,  and  freezes  into 
ice,  which  in  the  spring  produces  mud,  although  there  may  have  been  little 
rain  or  snow. 


ADHESION.  51 

attraction  between  the  solid  and  the  liquid.  Hopes 
absorb  water  by  imbibition,  swell,  and  shrink  often 
to  breaking.* 

4.  Solution. — Sugar  will  dissolve  in  water,  because 
the  adhesion  between  the  two  substances  is  stronger 
than  the  cohesion  of  the  sugar,  f  As  heat  weakens 
cohesion,  it  hastens  solution,  so  that  a  substance  gen- 
erally dissolves  more  rapidly  in  hot  water  than  in 
cold.  In  like  manner,  pulverizing  a  solid  aids  solu- 
tion. Liquids  also  absorb  gases  by  adhesion.  Thus 
water  contains  air,  which  renders  it  pleasant  to  the 
-taste.  As  pressure  and  cold  weaken  the  repellent 
force,  they  favor  the  adhesion  between  the  molecules 

*  It  is  1586.  The  Egyptian  obelisk,  weighing  a  million  pounds,  is  to  be 
raised  in  the  square  of  St.  Peter's,  Borne.  Pope  Sixtus  V.  proclaims  that  no 
one  shall  utter  a  word  aloud  until  the  engineer  announces  that  all  danger 
is  passed.  As  the  majestic  column  ascends,  all  eyes  watch  it  with  wonder 
and  awe.  Slowly  it  rises,  inch  by  inch,  foot  by  foot,  until  the  task  is  almost 
completed,  when  the  strain  becomes  too  great.  The  huge  ropes  yield  and 
slip.  The  workmen  are  dismayed,  and  fly  wildly  to  escape  the  impending 
mass  of  stone.  Suddenly  a  voice  breaks  the  silence.  "Wet  the  ropes,"  rings 
out  clear-toned  as  a  trumpet.  The  crowd  look.  There,  on  a  high  post,  stand- 
ing on  tiptoe,  his  eyes  glittering  with  the  intensity  of  excitement,  is  one  of 
the  eight  hundred  workmen,  a  sailor  named  Bresca  di  S.  Bemo.  His  voice 
and  appearance  startle  every  one;  but  his  words  inspire.  He  is  obeyed. 
The  ropes  swell  and  bite  the  stone.  The  column  ascends  again,  and  in  due 
time  stands  securely  on  its  pedestal.  The  daring  sailor  is  not  only  for- 
given, but  his  descendants  to  this  day  enjoy  the  reward  of  providing  the 
palm-branches  used  on  Palm  Sunday  at  St.  Peter's. 

t  This  contest  between  adhesion  and  cohesion  is  seen  when  we  let  fall 
on  water  a  drop  of  oil.  Adhesion  tends  to  draw  the  oil  to  the  liquid,  so  as 
to  mix  thoroughly,  and  cohesion  to  prevent  this.  The  extent  to  which  the 
drop  will  spread  will  depend  on  the  relation  of  the  two  attractions,  and  vary 
for  every  substance.  Thus  each  oil  has  its  own  COHESION  FIGTJBE,  which  en- 
ables the  chemist  readily  to  detect  differences  and  mixtures.—  Experiments: 
Dissolve  a  little  salt  in  a  glass  of  water,  and  touch  the  surface  of  the  liquid 
with  a  pen  full  of  ink.  The  characteristic  figures  will  quickly  appear.— Dis- 
solve in  water  a  pinch  of  salt  and  a  lump  of  loaf-sugar.  Touch  the  surface 
with  lunar  caustic.  Tha  figure  of  nitrate  of  silver  will  be  seen. 


52  ATTRACTION. 

of  a  gas  and  water.  Soda-water  receives  its  effer- 
vescence and  pungent  taste  from  carbonic-acid  gas, 
which,  being  absorbed  under  great  pressure,  escapes 
21  in  sparkling  bubbles  when  the  pressure  is 
removed. 

5.  Diffusion  of  Liquids. — Let   a   jar  be 

partly   filled  with  water  colored   by  blue 
litmus.    Then,  by  a  funnel-tube,  pour  clear 
water    containing    sulphuric    acid    to    the 
bottom,    beneath    the    colored    water.     At 
first,  the  two  will  be  distinctly  defined,  but 
Diffusion  of     in  a  few   days  they  will   mix,  as  will  be 
Liquids.        seen  "by  the  change  of  color  from  blue  to 
red.     A  drop  of  sulphuric  acid  may  thus  be  distrib- 
uted through  a  quart  of  water.    Most  liquids  will  min- 
gle when  brought  in  contact.    If,  however, 

TFrn       OO 

there  is  no  adhesion  between  their  mole- 
cules, they  will  not  mix,  and  will  separate 
even  after  having  been  thoroughly  shaken 
together.  For  example,  shake  together  mer- 
cury, water,  and  sweet  oil ;  they  will  soon  sep- 
arate and  settle  in  layers  with  the  oil  at  the 
top  and  the  mercury  at  the  bottom.  Diffu- 
sion is  a  slow  process,  so  we  generally  help  it  Diffusion  of 
by  shaking  or  stirring  the  mixture  of  liquids.* 

6.  Diffusion  of  Gases. — Hydrogen  gas  is  only  -fa  as 
heavy  as  common  air.  Yet,  if  two  bottles  be  ar- 
ranged as  in  Fig.  22,  the  lower  one  filled  with  the 

*  A  story  is  told  of  some  negroes  in  the  West  Indies  who  supplied  them- 
selves with  liquor  by  inverting  the  neck  of  a  bottle  of  water  in  the  bung- 


ADHESION. 


53 


FIG.  23. 


heavy  gas,  and  the  upper  with  the  lighter,  the  gases 
will  soon  be  uniformly  mixed.* 

7.  Osmose. — When  two  liquids  are  separated  by  a 
thin  substance,  the   interchange  may  be  modified  in 

hole  of  a  cask  of  rum.  The  water  sank  into  the  barrel,  -while  the  rum  rose 
to  take  its  place.  Water  and  rum  diffuse  readily,  "but  rum  is  lighter  and 
requires  time  to  diffuse  to  the  bottom. 

*  This  phenomenon  is  explained  by  the  theory  that  the  molecules  of  all 
bodies  are  in  rapid  motion  (p.  44,  §  2).  In  a  gas  the  molecules  are  assumed  to 
move  in  straight  lines,  collide  with  each  other,  rebound,  and  so  have  their  di- 
rections continually  changed.  The  paths  over  which  they  move  are  inconceiv- 
ably minute,  but  the  velocity  with  which  they  move  is  correspondingly  great. 
The  particles  of  ammonia  gas,  for  example,  are  flying  to  and  fro  at  the  rate 
of  twenty  miles  per  minute.  "  Could  we,  by  any  means,"  says  Prof.  Cooke, 
"turn  in  one  direction  the  actual  motion  of  the  molecules  of  what  we  call 
still  air,  it  would  become  at  once  a  wind  blowing  seventeen  miles  per  min- 
ute, and  exert  a  destructive  power  compared  with  which  the  most  violent 
tornado  is  feeble."— Invert  a  bottle  over  a 
lighted  candle,  and  the  oxygen  of  the  inclosed 
air  being  soon  consumed  the  flame  goes  out. 
Instead  of  the  bottle,  use  a  foolscap-paper  cone. 
There  will  be  an  interchange  of  gases  through 
the  pores  of  the  paper  and  the  light  will 
burn  with  moderate  freedom. 

Diffusion  of  Gases  is  still  more  strikingly 
shown  in  the  experiment  of  Pig.  23,  devised 
by  Prof.  Graham.  Mt  a  porous  cup  used  in 
Grove's  Battery  (p.  320)  with  a  cork  and  glass 
tube.  Fasten  the  tube  so  that  it  will  dip  be- 
neath the  colored  water  in  the  glass.  Then 
invert  over  the  cup  a  jar  of  hydrogen.  The 
gas  will  pass  through  the  sensible  pores  of 
the  earthenware  and  down  the  tube  so  rapidly, 
as  almost  instantly  to  bubble  up  through  the 
water.  Rose  balloons  lose  their  buoyancy,  be- 
cause the  hydrogen  escapes  through  the  pores 
of  the  rubber.  If  they  were  filled  with  air  and 
placed  in  a  jar  of  hydrogen,  that  gas  would 
creep  in  so  rapidly  as  to  burst  them.— In  per- 
forming the  experiment  shown  in  Kg.  23, 
coal-gas  may  be  used.  After  the  bubbling 

ceases,  on  withdrawing  the  jar  water  rises  in  the  tube.  The  thin  gas 
diffuses  out  into  the  denser  air,  producing  a  partial  vacuum  within. 


Graham's  Diffusion  Tube. 


54 


ATTRACTION. 


FIG.  34. 


a  curious  manner,  according  to  the  nature  of  the 
liquid  and  the  substance  used.  At  the  end  of  a  glass 
tube  (Fig.  24)  fasten  a  bladder  filled  with  alcohol. 
Insert  into  a  jar  of  water,  and 
mark  the  height  to  which  the 
alcohol  ascends  in  the  tube.  The 
column  will  soon  begin  to  rise 
slowly.  On  examination,  we  shall 
see  that  the  alcohol  is  passing  out 
through  the  pores  of  the  blad- 
der and  mixing  with  the  water, 
while  the  water  is  coming  in 
more  rapidly.  The  bladder  is 
not  porous  in  the  sense  of  hav- 
ing sensible  pores. 

Diffusion  of  fluids,  through 
the  medium  of  a  substance 
which  attracts  them  unequally, 
is  called  osmose.  It  is  applied 
by  the  chemist  in  methods  of 
analysis  where  the  separation  of  substances  is  based 
on  their  unequal  diffusibility.  Crystals  in  solution 
pass  readily  through  animal  membrane,  like  bladder, 
while  substances  that  do  not  crystallize,  like  gum, 
gelatine,  or  white  of  egg,  are  stopped. 


Osmose. 


PRACTICAL   QUESTIONS. 

1.  Why  does  cloth,  shrink  when  wet  ? 

2.  Why  do  sailors  at  a  boat-race  wet  the  sails  ? 

3.  Why  is  writing-paper  sized  ? 

4.  Why  does  paint  prevent  wood  from  shrinking  f 


ATTRACTION     OF     GRAVITATION.  55 

5.  What  is  the  shape  of  the  surface  of  a  glass-full  of  water?  Of  mercury? 

6.  "Why  can  we  not  perfectly  dry  a  towel  by  wringing  ? 

7.  Why  will  not  water  run  through  a  fine  sieve  when  the  wires  are 
greased? 

8.  Why  will  camphor  dissolve  in  alcohol,  and  not  in  water? 

9.  Why  will  mercury  rise  in  zinc  tubes  as  water  will  in  glass  tubes? 

10.  Why  will  ink  spilled  on  the  edge  of  a  book  extend  farther  inside 
than  if  spilled  on  the  side  of  the  leaves? 

11.  If  you  should  happen  to  spill  some  ink  on  the  edge  of  your  book, 
ought  you  to  press  the  leaves  together? 

12.  Why  can  you  not  mix  water  and  oil? 

13.  What  is  the  object  of   the    spout  on  a  pitcher?     Ans.  The  water 
would  run  down  the  side  of  the  pitcher  by  the  force  of  adhesion,  but  the 
spout  throws  it  into  the  hands  of  gravitation  before  adhesion  can  catch  it. 

14.  Why  will  water  wet  your  hand,  while  mercury  will  not? 

15.  Why  is  a  pail  or  tub  liable  to  fall  to  pieces  if  not  filled  with  water 
or  kept  in  a  damp  place? 

16.  Name  instances  where  the  attraction  of.  adhesion  is  stronger  than 
that  of  cohesion. 

17.  Why  does  the  water  in  Pig.  18   stand  higher  inside  of  the  tube 
than  next  the  glass  on  the  outside? 

18.  Why  will  clothes-lines  tighten  and  sometimes  break  during  a  shower? 

19.  In  casting  large  cannon,  the  gun  is  cooled  by  a  stream  of  cold 
water.    Why  ? 

20.  Why  does  paint  adhere  to  wood?    Chalk  to  the  blackboard? 

21.  Why  does  a  towel  dry  one's  face  after  washing? 

22.  Why  will  a  greased  needle  float  on  water? 

23.  Why  is  the  point  of  a  pen  slit? 

24.  Why  is  a  thin  layer  of  glue  stronger  than  a  thick  one? 


II.    ATTRACTION   OF   GRAVITATION. 

WE  have  spoken  of  the  attraction  existing  between 
the  molecules  of  bodies  at  minute  distances.  We  now 
notice  an  attraction  which  acts  at  all  distances. 

1.  Law  of  Gravitation. — Hold  a  stone  in  the  hand, 
and  you  feel  a  power  constantly  drawing  it  to  the 
ground.  "We  call  this  familiar  phenomenon  weight. 
It  is  really  the  attraction  of  the  earth  pulling  the 


56 


ATTK  ACTION. 


PIG.  25. 


stone  back  to  itself — an  instance  of  a  general  law, 
one  operation  of  an  ever-active  force.  For  every  par- 
ticle of  matter  in  the  universe*  attracts  every  other 
particle  with  a  force  proportional  to  the  product  of 
their  masses,  and  increasing  as  the  square  of  the  dis- 
tance decreases. 

Gravitation  is  the  general  term  applied  to  the 
attraction  that  exists  between  all 
bodies  in  the  universe.  Gravity  is 
the  earth's  attraction  for  terrestrial 
bodies;  it  tends  to  draw  them  to- 
ward the  center  of  the  earth. 

2.  Illustrations   of  Gravity. — A 

stone  falls  to  the  ground  because  the 
earth  attracts  it;  but  in  turn  the 
stone  attracts  the  earth.  Each  moves 
to  meet  the  other,  but  the  stone 
passes  through  as  much  greater  dis- 
tance than  the  earth  as  its  mass  is 
less.  The  mass  of  the  earth  is  so 
great  that  its  motion  is  impercepti- 

Deflection  of   a  Plumb-     ' 

line  by  a  mountain,    ble. — A  plumb-line    hanging  near  a 

(Exaggerated.)  .        . 

mountain  is  attracted  from  the  ver- 
tical.   In  Fig.  25,  AB  represents  the  ordinary  posi- 

*  The  force  of  gravitation  acts  on  every  particle  of  matter,  and  hence 
it  is  not  confined  to  our  own  world.  By  its  action  the  heavenly  bodies  are 
bound  to  one  another,  and  thus  kept  in  their  orbits.  It  may  help  us  to  con- 
ceive how  the  earth  is  supported,  if  we  imagine  the  sun  letting  down  a 
huge  cable,  and  every  star  in  the  heavens  a  tiny  thread,  to  hold  our  globe 
in  its  place,  while  it  in  turn  sends  back  a  cable  to  the  sun  and  a  thread  to 
every  one  of  the  stars.  So  we  are  bound  to  them  and  they  to  us.  Thus  the 
worlds  throughout  space  are  linked  together  by  these  cords  of  mutual  attrac- 
tion, which,  interweaving  in  every  direction,  make  the  universe  a  unit. 


r 


ATTRACTION     OF     GRAVITATION.  57 

tion  of  the   line,  while  AC  indicates  the   attractive 
power  (greatly  exaggerated)  of  the  mountain.* 

3.  Mass  and  Weight. — The  quantity  of  matter  in 
a  body  is  its  mass.    The  measure  of  the  earth's  at- 
traction upon  it  is  its  weight.    If  m  be  the  number 
of  units  of  mass  in  a  body,  and  g  be  the  number  of 
units  of  force  expressing  the  earth's  attraction,  then 
its   weight,   w,  is   equal  to   m  multiplied  by  g;  or, 

w  =  mg,  whence 

w 

m  =  — . 
9 

4.  The  Earth's  Center  of  Gravity  is  that  point 
within  it  where   the   attraction   of  all  the   particles 
on   any   one   side  is   equal  to   the   attraction   of  all 
those  on  the  opposite  side.    As  an  attracting  mass 
the  whole  earth  may  be  regarded  as  if  it  were  con- 
centered at  this  point.    Its  position  is  probably  very 
near  the  geometric  center.    When  the  earth's  center 
is  mentioned  we  generally  mean  its  center  of  gravity. 

5.  The  Center  of  Gravity  of  a  Body  is  that  point 
about  which   it   may  be   balanced.    A   straight   line 
from  this  point  to  the  earth's   center   of   gravity  is 
called  the  line  of  direction.    It  is  also  called  a  verti- 
cal, or  plumb-line.    Gravity  tends  to  cause  the  body 
to  move  along  this  line  toward  the  center. f 

*  Maskelyne,  in  1774,  found  the  attraction  of  Mount  Schehallien  to  de- 
flect a  plumb  line  12".  By  comparing  this  force  with  that  of  the  earth,  the 
specific  gravity  of  the  earth  was  estimated  to  be  five  times  that  of  water. 
Later  investigations  make  it  5.67. 

t  Downward  means  toward  the  earth's  center;  upward  means  the  oppo- 
site. Any  two  bodies  moving  downward,  one  from  America  and  the  other 
from  Europe,  if  unresisted,  would  meet  at  the  earth's  center  if  their  fall 
were  properly  timed. 


58  ATTRACTION. 

6.  Laws  of  Weight.  —  I.  The  weight  of  a  body  at 
the  center  of  the  earth  is  nothing  ;  for  since  the  oppo- 
site attractions  are   mutually  balanced  there  can  be 
no  tendency  to  motion  in  any  direction. 

II.  The  weight  of  a  ~body  above  the  surface  of  the 
earth  decreases  as   the  square  of  the  distance  from 
the  center  of  the  earth  increases* 

III.  The  weight  of  a  body  varies  on  different  por- 
tions of  the  surface  of  the  earth.\    It  will  be  least  at 
the  equator,  because  (1),  on  account  of  the  bulging 
form  of  our  globe  a  body  is  there    farther   from  the 
earth's  center;  and  (2),  the  centrifugal  force  is  there 
strongest.    It  will  be  greatest   at  the  poles,  because 
(1),  on  account  of  the  flattening  of  the  earth  a  body 
at  a  pole  is  there  nearer  to  the  earth's  center;  (2), 
there  is  no  centrifugal  force  at  the  poles. 

7.  Falling  Bodies.  —  I.  Under  the  influence  of  the 
constant  force  of  gravity  alone  all  bodies  fall  with 
equal  rapidity. 

*  A  body  at  the  surface  of  the  earth  (4,000  miles  from  the  center)  weighs 
100  Ibs.  What  would  be  its  weight  1,000  miles  above  the  surface  (5,000  miles 
from  the  center)?  SOLUTION:  (5,000  mi.)2  :  (4,000  mi.)2  ::  100  Ibs.  :x  =  64 

Ibs,    Or,  its  weight  would  decrease  in  the  ratio  of         ™  =  if-    Hence  it 


Would  weigh  £f  x  100  Ibs.  =  64  Ibs.—  The  weight  of  a  body  below  the  surface 
of  the  earth  is  commonly  said  to  decrease  directly  as  the  distance  from  the 
center  decreases.  Thus,  1,000  miles  below  the  surface,  a  body  would  lose 
J  its  weight.  In  fact,  however,  the  density  of  the  earth  increases  so  much 
toward  the  center,  that  "  for  TT5  of  the  distance  the  force  of  gravity  actually 
becomes  stronger  than  on  the  surface." 

t  In  these  statements  concerning  weight,  a  spring-balance  is  supposed 
to  be  employed.  If  it  be  graduated  to  indicate  correctly  at  a  medium  lati- 
tude, it  would  show  too  little  at  the  equator,  and  too  much  at  the  poles. 
In  other  words,  a  pound  weighed  with  such  a  spring-balance  at  the  equator 
would  contain  a  greater  mass  of  matter  than  one  weighed  at  the  poles  by 
about  T*5  nart. 


FALLING     BODIES. 


59 


FIG.  26. 


This  is  well  illustrated  by  the  "  guinea  and  feather 
experiment."  Let  a  coin  and  a  feather  be  placed  in 
a  tube,  and  the  air  exhausted. 
Quickly  invert  the  tube,  and  the 
two  bodies  will  fall  in  nearly  the 
same  time.  Let  in  the  air  again, 
and  the  feather  will  nutter  down 
long  after  the  coin  has  reached 
the  bottom.*  Hence  we  conclude 
that  in  a  vacuum  all  bodies  de- 
scend with  equal  velocity,  and 
that  the  resistance  of  the  air  and 
the  adhesion  of  the  feather  to 
the  tube  are  the  causes  of  the 
variation  we  see  between  the  fall- 
ing of  light  and  of  heavy  bodies 
in  it. 

II.  AT  WOOD'S  MACHINE.! — To 
deduce  the  laws  of  falling  bodies 
we  make  use  of  Atwood's  ma- 
chine (Fig.  27).  This  consists  of 
a  very  light  grooved  wheel,  w, 


Guinea  and  Feather  Ex- 
periment. 


*  The  same  fact  may  be  noticed  in  the  case  of  a  sheet  of  paper.  When 
spread  out,  it  merely  nutters  to  the  ground ;  but  when  rolled  in  a  compact 
mass,  it  falls  quickly.  In  this  case  we  have  not  increased  the  force  of  at- 
traction, but  we  have  diminished  the  resistance  of  the  air.  "It  is  difficult 
for  many  pupils  to  understand  how,  under  the  influence  of  gravity  alone, 
all  bodies  fall  with  equal  rapidity.  An  illustration,  which  is  usually  effect- 
ive, is  that  of  a  number  of  bodies  of  the  same  kind,  say  bricks,  which  will 
separately  fall  in  the  same  space  of  time.  The  pupil  will  admit  that,  if  all 
of  them  are  connected  together,  inasmuch  as  nothing  is  thereby  added  to 
their  weight,  there  is  no  reason  why  the  mass  of  bricks  should  not  fall  in  the 
time  of  a  Dingle  one,  notwithstanding  it  is  a  larger  body."— WM.  H.  TAYLOB. 

t  If  the  ieacher  is  not  provided  with  Atwood's  machine,  or  if  the  pupils 


ATTRACTION. 


Fro.  27. 


Atwood's  Machine. 


pivoted  at  the  top  of  a  firm 
vertical  pillar,  on  one  side 
of  which  is  a  graduated 
strip,  s,  divided  into  inches 
or  centimeters.  A  silk 
thread,  passing  over  the 
wheel,  supports  two  equal 
masses,  m  and  m'.  The 
force  of  gravity  on  one 
of  these  just  balances  its 
force  on  the  other.  A 
small  cross-bar,  n,  placed 
upon  m,  gives  vertical  mo- 
tion, by  the  action  of  grav- 
ity on  it,  to  the  whole 
mass,  which  we  may  call 
M}  made  up  of  (m  -f-  m' 
+  n).  Since  the  momen- 
tum of  m,  moving  down- 
ward, is  balanced  by  that 
of  m',  moving  upward,  the 
rate  of  motion  of  M  is  as 
much  less  than  if  it  were 
falling  freely  as  the  mass 
of  the  cross-bar  is  less  than 

are  unfamiliar  with,  algebra,  it  may 
be  found  best  to  omit  this  discussion 
of  Falling  Bodies.  The  subject  re- 
ceives special  attention  in  the  present 
edition  on  account  of  the  united  re- 
quest of  many  teachers.  It  has  been 
reduced  within  the  narrowest  limit 
consistent  with  clearness. 


FALLING     BODIES.  61 

the  whole  mass,  M.  An  allowance  has  to  be  made 
for  the  mass  of  the  wheel,  which  is  put  in  circular 
motion  at  the  same  time.  By  properly  choosing  the 
mass  of  the  cross-bar,  the  motion  of  M  may  be 
made  so  slow  that  the  distance  through  which  it 
passes  in  each  succeeding  second  may  easily  be 
measured  on  the  graduated  strip  along  which  m  falls. 
A  pendulum,  o,  marks  the  successive  seconds,  and  is 
so  arranged  as  to  release  the  support  of  m  at  the  proper 
moment,  thus  allowing  M  to  move.  Attached  to  the 
graduated  strip  are  a  movable  ring,  r,  and  a  movable 
plate,  p.  If  these  be  placed  as  shown  in  Fig.  27,  the 
cross-bar  would  be  caught  by  the  ring,  while  m  pass- 
ing through  would  continue  to  move  until  stopped 
by  the  plate  p. 

EXPERIMENTS  WITH  AT  WOOD'S  MACHINE. —  Suppose 
the  mass  of  the  cross-bar  be  so  adjusted  to  that  of  m 
and  m'  that  when  m  is  released  by  the  pendulum  it 
will  pass  through  5  cm.  (p.  4)  during  the  first  sec- 
ond. Let  the  plate,  _p,  be  removed,  and  the  ring,  r, 
be  placed  5  cm.  below  the  starting-point  of  m.  Then 
allowing  the  system  M  to  fall,  the  cross-bar  will  be 
caught  by  the  ring  exactly  at  the  end  of  one  second 
and  stopped;  the  remainder  of  the  system,  however, 
will  continue  its  motion  (p.  22,  §  4).  m  passing 
through  the  ring  will  be  seen  to  pass  over  1 0  cm.  in 
each  successive  second  until  it  reaches  the  foot  of 
the  instrument.  10  cm.  per  second,  then,  is  the  ve- 
locity produced  by  the  force  of  gravity  acting  for 
one  second  on  the  cross-bar.  Let  the  ring,  for  a  sec- 
ond experiment,  be  placed  20  cm.  below  the  starting- 


62 


ATTRACTION. 


point. 


FIG.  28. 


I  6 

a 
to 


On  allowing  the  system  to  fall,  the  cross-bar 
will  now  be  caught  exactly  at  the 
end  of  two  seconds,  m  passing 
through  the  ring  will  be  seen  to 
move  with  a  velocity  of  2  0  cm.  per 
second.  We  have  evidently  doubled 
the  velocity  by  allowing  the  force  of 
gravity  to  act  for  two  seconds  on  the 
cross-bar.  Similarly,  by  allowing 
gravity  to  act  for  three  seconds, 
the  velocity  will  be  trebled.  The 
longer,  therefore,  the  force  is  al- 
lowed to  act  the  greater  will  be  the 
velocity  produced.  The  distances 
passed  over  during  the  successive 
seconds  were  5,  15,  25  cm.  (see  Fig. 
28).  Under  the  action  of  a  constant 
force,  therefore,  a  body  will  move 
with  a  continually  increasing  ve- 
locity. The  gain  in  velocity  per  unit 
of  time  is  called  the  -acceleration, 
and  the  motion  itself  is  described  as 
accelerated  motion.  If  the  accelera- 
tion for  successive  seconds,  as  in  our 
experiments,  has  the  same  numeri- 
cal value,  the  motion  is  called  uni- 
formly accelerated  motion,  and  this 
is  the  motion  of  a  falling  body. 

From  the  first  experiment  it  is 
seen  that  during  the  first  second  the 
system  M  passed  over  5  cm.,  but  that  the  acceleration 


ATTRACTION. 


63 


produced  in  that  time  was  double  this,  or  1 0  cm.  per 
second.  A  body,  therefore,  under  the  action  of  a  con- 
stant force  and  starting  from  a  state  of  rest  passes  over 
a  distance  during  the  first  second  equal  to  one  half  the 
acceleration  produced  during  that  time. 

On  repeating  the  experiments  with  cross-bars  of 
greater  mass,  it  will  be  found  that  the  accelerations 
produced  increase  with  the  mass  of  the  cross-bar, 
that  is,  with  the  force  employed.  A  constant  force 
may,  therefore,  be  measured  by  the  acceleration  it 
imparts  to  unit  mass  (page  23). 

EQUATIONS  OF  FALLING  BODIES. — These  results  are 
combined  in  the  accompanying  table,  where  the  ac- 
celeration, 10  cm.,  is  represented  by/. 

FIG.  29. 


t 

1 

2 

3 

4 

V 

/ 

/x2 

/X3 

/x4 

*=/*                       (1) 

s 

M 

i/x3 

|/X5 

I/X7 

S=i/(2*-l)             (2) 

8 

if 

i/x4 

i/x9 

i/xl6 

S=|/*2                   (3) 

The  horizontal  line  marked  t  gives  the  time  in 
seconds;  that  marked  v,  the  velocity  at  the  end  of 
each  second;  that  marked  s,  the  space  traversed 
during  each  successive  second ;  and  that  marked  S, 
the  whole  space  traversed  from  the  beginning  of  fall 
to  the  end  of  each  second.  Taking  any  one  of  the 
vertical  columns,  such  as  the  fourth,  we  see  that 

(1.)  At  the  end  of  any  given  second,  the  velocity 


64  ATTRACTION. 

is  equal  to  the  acceleration  multiplied  by  the  num- 
ber of  the  second  ;  or,  v  —  ft. 

(2.)  During  any  particular  second,  the  space  trav- 
ersed is  equal  to  half  the  acceleration  multiplied  by 
one  less  than  double  the  number  of  the  second  ;  or, 
8  =  tf(M-  I). 

(3.)  The  whole  space  traversed  during  a  given 
number  of  seconds  is  equal  to  half  the  acceleration 
multiplied  by  the  square  of  the  time  in  seconds;  or, 
8  = 


8.  Bodies  Falling  Freely.—  If  a  body  falls  freely, 
the  acceleration  is  about  9.8  meters,  or  32  feet.*  If 
we  represent  this  by  g  in  the  equations  just  deduced, 
we  have 


.............. 

a  =  $g(2t  -  1)  ..........  (2) 


(3)f 


When  a  body  is  thrown  upward,  gravity  causes  it 
to  lose  32  feet  in  velocity  each  second  until  it  ceases 
to  ascend.  The  velocity  with  which  it  begins  to 
rise,  and  its  time  of  rising,  must  be  the  same  as  the 
velocity  with  which  it  ends  its  fall,  and  the  time  of 
falling.  The  laws  of  falling  bodies  may  hence  be 
applied. 

*  For  the  latitude  of  New  York,  g  =  32.16  more  nearly,  or  980.2  cm. 
This  varies  slightly  with  the  latitude  and  elevation  of  a  place;  but  for 
ordinary  problems  it  will  be  sufficient  to  assume  the  values  given  in  the 
text. 

t  An  additional  formula  that  is  often  useful  may  be  obtained  by  elim- 
inating t  in  combining  equations  (1)  and  (3).  The  result  is  v  =  V2gS. 
Since  8  represents  height  (h),  it  is  often  expressed  t?3  =  2gh.  (4). 


EQUILIBRIUM.  65 

9.  Measurement  of  Kinetic  Energy.  (See  p.  84.) 
—  To  lift  a  body  through  any  height  energy  must 
be  expended.  The  measure  of  this  is  the  weight 
(w),  multipled  by  the  height  (h),  to  which  it  is  lifted. 
Using  the  initial  letters  of  these  words  for  symbols, 

we  have 

K=wh. 

But  from  the   equation,  at  the  bottom  of  p.  64, 

v2 
we  have  h  =  ~-.  And  from  p.  57,  w  =  mg.  Sub- 

^9 
stituting  these  values  and  reducing,  we  have 


1C.   Resistance  of  the  Air  to  Moving  Bodies.  —  A 

body  in  falling  or  otherwise  moving  through  the  air 
expends  energy  in  overcoming  the  resistance  of  the 
air.  From  the  formula  just  deduced  we  see  that 
this  is  proportional  to  the  mass  of  air  moved  and  to 
the  square  of  the  velocity.  The  flight  of  a  cannon- 
ball  is  never  so  great  as  it  would  be  if  shot  through 
a  vacuum.  Practically  it  is  not  easy  to  calculate 
beforehand  the  amount  of  energy  to  be  lost  through 
resistances. 

11.  Equilibrium.  —  When  a  body  is  at  rest  the 
forces  which  act  on  every  molecule  in  it  are  said  to 
balance  one  another,  or  to  be  in  equilibrium.  The 
most  important  of  these  forces  is  gravity. 

(1.)  THREE  STATES  OF  EQUILIBRIUM.  —  1st.  A  body 
is  in  stable  equilibrium  when  the  center  of  gravity 
is  below  the  point  of  support,  or  when  any  move- 
ment tends  to  raise  the  center  of  gravity.  In  Fig.  30, 


66  ATTRACTION, 

the  cork  and  two  knives  together  form  a  connected 
body  whose    center    of   gravity  is    outside,  just    be- 
neath  the   needle.      By  pushing   either  knife  a  few 
oscillations  are  produced,  but  a  posi- 

FIG.  30. 

tion  of  rest  is  soon  recovered.  Any 
movement  of  the  toy  shown  in  Fig. 
31  tends  to  raise  the  center  of  grav- 
ity, and  it  returns  quickly  to  a  state 
of  rest. 

2d.  A  body  is  said  to  be  in  un- 

Stable  Equilibrium.  ,  -,  .7 .-,      .  ,  ,, 

stable   eqmhomum    when    the   center 
of  gravity  is  above   the  point  of  support,  or  when 
any  movement  tends  to  lower  the  center  of  gravity. 
If  we  take  the  cork  as  arranged  with 
the  knives  in  Fig.  30,  and  invert  it,  Fl0' 31< 

we  shall  have  difficulty  in  balancing 
the  needle ;  and,  if  we 
succeed,  it  will  readily 
topple  off,  as  the  least 
motion  tends  to  lower  the 
center  of  gravity. 

3d.  A  body  is  said  to 
be  in  indifferent  equilib- 
rium when  the  center  of 
gravity  is  at  the  point  of 

Stable  Equilibrium. 

support,    or    when    any 

movement   tends   neither  to   elevate   nor  lower  the 

center  of  gravity.     A  ball  of  uniform  density  on  a 

level  surface  will  rest  in  any  position,  because  the 

center  of  gravity  moves    in  a  line   parallel  to   the 

floor. 


EQUILIBRIUM. 


67 


(2.)  GENERAL  PRINCIPLES. — (a.)  The  center  of  grav- 
ity tends  to  seek  the  lowest  point. 

(u)  A  body  will  not  tip  over  while  the  line  of 
direction  falls  within  the  base,  but  will  as  soon  as  it 
falls  without.* 

(c.)  In  general,  narrowness  of  base  combined  with 
height  of  center  of  gravity,  tends  to  instability ;  f 
breadth  of  base  and  lowness  of  center  of  gravity, 
produce  stability. 

(3.)  PHYSIOLOGICAL  FACTS. — Our  feet  and  the  space 
between  them  form  the  base  on  which  we  stand. 
By  turning  our  toes  outward,  we  increase  its  breadth. 


Fro. 


*  The  Leaning  Tower  of  Pisa,  in  Italy,  beautifully  illustrates  this  prin- 
ciple.    It  is  about  188  feet  high,  and  its  top  leans  15  feet,  yet  the  line  of 
direction  falls  so  far  within  the  base  that  it 
is  perfectly  stable,  having  stood  for  seven  cent- 
uries.    The  feeling  experienced  by  a  person 
who  for  the  first  time  looks  down  from  the 
lower  side  of  the  top  of  this  apparently  im- 
pending structure  is  startling  indeed. 

t  "  This  is  shown  by  the  difficulty  in  learn- 
ing to  walk  upon  stilts.  The  art  of  balancing 
one's  self  may,  however,  be  acquired  by  prac- 
tice, as  is  seen  in  the  Landes  of  south-western 
France.  During  a  portion  of  the  year  these 
sandy  plains  are  half  covered  with  water,  and 
in  the  remainder  are  still  very  bad  walking. 
The  natives  accordingly  double  the  length  of 
their  legs  by  stilts.  Mounted  on  these  wooden 
poles,  which  are  put  on  and  off  as  regularly 
as  the  other  parts  of  their  dress,  they  appear 
to  strangers  as  a  new  and  extraordinary  race, 
marching  with  steps  of  six  feet  in  length, 
and  with  the  speed  of  a  trotting-horse.  While 
watching  their  flocks,  they  support  themselves 
"by  a  third  staff  behind,  and  then  with  their  Walking  on  Stats, 

rough  sheep-skin  cloaks  and  caps,  like  thatched 

roofs,  seem  to  be  little  watch-towers,  or  singular  lofty  tripods,  scattered 
over  the  country."— ABNOTT. 


68 


ATTRACTION. 


FIG.  83. 


When  we  stand  on  one  foot,  we  bend  over  so  as  to 
bring  the  line  of  direction  within  this  narrower  base. 
When  we  walk,  we  incline  to  the  right  and  the  left 
alternately.  When  we  walk  up  hill  we  lean  forward, 
and  in  going  down  hill  we  incline  backward,  in  un- 
conscious obedience  to  the  laws  of  gravity.  We  bend 
forward  when  we  wish  to  rise  from  a  chair,  in  order 
to  bring  the  center  of  gravity  over  our  feet.  In  walk- 
ing we  lean  forward,  so  as  bring  the  center  of  gravity 
as  far  in  front  as  possible.  Thus,  walking  is  a  pro- 
cess of  falling  forward  and  then  checking  the  fall. 
When  we  run,  we  lean  farther  forward,  and  so  fall 
faster.  ("Hygienic  Physiology/'  p.  37.) 

12.  The  Pendulum  consists  of  a  weight  so  sus- 
pended as  to  swing  freely.  Its  move- 
ments to  and  fro  are  termed  vibra- 
tions or  oscillations.  The  path  through 
which  it  passes  is  called  the  arc.  The 
extent  to  which  it  goes  in  either  direc- 
tion from  the  lowest  point  is  styled 
its  amplitude.  Vibrations  performed 
in  equal  times  are  termed  i-soch'ro- 
nous  (isos,  equal;  chronos,  time). 

(1.)   THREE  LAWS. — I.   In  the  same 
pendulum,  all  vibrations  of  small  am- 
plitude are  isochronous.    If  we  let  one 
of    the    balls    represented    in   Fig.    3  3 
swing  through  a  short  arc,  and  then 
Pendulums.         through  a  longer  one,  on  counting  the 
number  of  oscillations  per  minute,  we  shall  find  them 
very  uniform. 


THE     PENDULUM. 


69 


Fw.  34. 


II.  The  times  of  the  vibrations  of  different  pendu- 
lums are  proportional  to  the  square  roots  of  their  re- 
spective   lengths.  —  Example  : 

A  pendulum  -J  the  length  of 
another,  will  vibrate  three 
times  as  fast.*  Conversely, 
the  lengths  of  different  pen- 
dulums are  proportional  to 
the  squares  of  their  times  of 
vibration. 

III.  The  time  of  the  vibra- 
tion  of  the  same  pendulum 
will  vary  at  different  places, 
since    it   decreases    as   the 
square  root  of  the  number  ex- 
pressing  the  acceleration   of 
gravity    increases.      At    the 
equator  a  pendulum  vibrates 
most  slowly.     The  length  of 

a    Seconds-pendulum    at    New   Pendulums   of  apparently   the  same 
«.  .  ,     .         ,  length,  but  really  different  lengths. 

York  is  about  39-^  inches. 

(2.)   CENTER  OF  OSCILLATION.  —  The  upper  part  of  a 

pendulum  tends  to  move  faster  than  the  lower  part, 

f 

*  A  pendulum  which  vibrates  seconds  must  be  four  times  as  long  as 
one  which  vibrates  half-seconds.  The  apparatus  represented  in  Figs.  33  and 
34  can  be  made  by  any  carpenter  or  ingenious  pupil,  and  will  serve  excel- 
lently to  illustrate  the  three  laws  of  the  pendulum.  The  law  of  the  pendu- 
lum may  be  conveniently  expressed  in  symbols.  If  t  be  the  time  of  a  sin- 
gle vibration  in  seconds,  /  the  length  of  the  pendulum,  g  the  acceleration 
of  gravity,  I  and  g  being  expressed  in  feet,  or  in  meters,  and  if  tr  be  the 
ratio  of  the  circumference  to  the  diameter  of  a  circle,  then 


This  formula  is  convenient  for  use  in  solving  problems. 


70  ATTRACTION. 

and  so  hastens  the  speed.  The  lower  part  of  a  pen- 
dulum tends  to  move  slower  than  the  upper  part, 
and  so  retards  the  speed.  Between  these  extremes 
is  a  point  which  is  neither  quickened  nor  impeded 
by  the  rest,  but  moves  in  the  same  time  that  it 
would  if  it  were  a  particle  swinging  by  an  imaginary 
line.  This  point  is  called  the  center  of  oscillation. 
It  lies  a  little  below  the  center  of  gravity.*  In  Fig. 
34  is  shown  an  apparatus  containing  pendulums  of 
different  shapes,  but  of  the  same  length.  If  they  are 
started  together,  they  will  immediately  diverge,  no 
two  vibrating  in  the  same  time.  As  pendulums,  they 
are  not  of  the  same  length. 

(3.)  THE  CENTER  OF  OSCILLATION  is  FOUND  BY 
TRIAL,  f — Huyghens  discovered  that  the  point  of  sus- 
pension and  the  center  of  oscillation  are  interchange- 
able. If,  therefore,  a  pendulum  be  inverted,  and  a 

*  This  determines  the  real  length  of  a  pendulum,  which  is  the  distance 
from  the  point  of  support  to  the  center  of  oscillation.  The  imaginary  pen- 
dulum above  described  is  known  in  Physics  as  the  Simple  Pendulum.— 39.1 
inches  =  993.3  mm. 

t  "  Take  a  flat  board  of  any  form  and  drive  a  piece  of  wire  through  it 
near  its  edge,  and  allow  it  to  hang  in  a  vertical  plane,  holding  the  ends  of 
the  wire  by  the  finger  and  thumb.  Take  a  small  bullet,  fasten  it  to  the 
end  of  a  thread,  and  allow  the  thread  to  pass  over  the  wire  so  that  the 
bullet  hangs  close  to  the  board.  Move  the  hancl  by  which  you  hold  the 
wire  horizontally  in  the  plane  of  the  board,  and  observe  whether  the  board 
moves  forward  or  backward  with  respect  to  the  bullet.  If  it  moves  forward, 
lengthen  the  string ;  if  backward,  shorten  it  till  the  bullet  and  the  board 
move  together.  Now  mark  the  point  of  the  board  opposite  the  center  of 
the  bullet,  and  fasten  the  string  to  the  wire.  You  will  find  that,  if  you  hold 
the  wire  by  the  ends  and  move  it  in  any  manner,  however  sudden  and 
irregular,  in  the  plane  of  the  board,  the  bullet  will  never  quit  the  marked 
spot  on  the  board.  Hence  this  spot  is  called  the  center  of  oscillation,  be- 
cause, when  the  board  is  oscillating  about  the  wire  when  fixed,  it  oscillates 
as  if  it  consisted  of  a  single  particle  placed  at  the  spot.  It  is  also  called 
the  center  of  percussion,  because,  if  the  board  is  at  rest  and  the  wire  is 


THE     PENDULUM. 


71 


FIG.  35. 


point  found  at  which  it  will  vibrate  in  the  same  time 
as  before,  this  is  the  former  center  of  oscillation ;  while 
the  old  point  of  suspension  becomes 
the  new  center  of  oscillation.* 

(4.)  THE  PENDULUM  AS  A  TIME- 
KEEPER.— The  friction  at  the  point  of 
suspension,  and  the  resistance  of  the 
air,  soon  destroy  the  motion  of  the 
pendulum.  The  clock  is  a  machine 
for  keeping  up  the  vibration  of  the 
pendulum,  and  counting  its  beats. 
In  Fig.  35,  R  is  the  scape-wheel 
driven  by  the  force  of  the  clock- 
weight  or  spring,  and  mn  the  escape- 
ment, moved  by  the  forked  arm,  AB, 
so  that  only  one  cog  of  the  wheel 
can  pass  at  each  double  vibration  of 
the  pendulum.  Thus  the  oscillations 
are  counted  by  the  cogs  on  the  wheel, 
while  the  friction  and  the  resistance 
of  the  air  are  overcome  by  the  ac- 
tion of  the  weight  or  spring,  f  As 
"  heat  expands  and  cold  contracts,". 

suddenly  moved  horizontally,  the  board  will  at  first 
begin  to  rotate  about  the  spot  as  a  center."  — J. 
CLEBK  MAXWELL,  on  "  Matter  and  Motion,"  p.  104. 

*  The  center  of  oscillation  is  the  same  as  the 
center  of  percussion.  The  latter  is  the  point  where  we 
must  strike  a  suspended  body,  if  we  wish  it  to  re- 
volve about  its  axis  without  any  strain.  If  we  do 
not  hit  a  ball  on  the  bat's  center  of  percussion,  our  Clock  Pendulum, 
hands  "  sting"  with  the  jar. 

t  The  action  of  a  clock  is  clearly  seen  by  procuring  the  works  of  an  old 
clock  and  watching  the  movements  of  the  various  parts. 


72  ATTRACTION. 

a  pendulum  lengthens  in  summer  and  shortens  in 
winter.     A   clock,   therefore,   tends  to   lose   time   in 
summer  and  gain  in  winter.     To  regulate  a  clock, 
we  raise  or  lower  the  pendulum-bob,  L,  by  the  nut  v. 
(5.)  OTHER  USES  OF  THE  PENDULUM. — (a.)  Since  the 
time  of  vibration  of  a  pendulum  indicates  the  force 
FIG  gg  of  gravity,   and  the   force 

of  gravity  decreases  as  the 
square  of  the  distance  from 
the  center  of  the  earth  in- 
creases, we  may  thus  find 
the  semi-diameter  of  the 
earth  at  various  places,  and 
ascertain  the  figure  of  our 
globe.  (&.)  Knowing  the 

Foucault's  Method.  ., 

force    of    gravity    at    any 

point,  the  velocity  of  a  falling  body  can  be  deter- 
mined, (c.)  The  pendulum  may  be  used  as  a  stand- 
ard of  measures,  (d.)  Foucault  devised  a  method 
of  showing  the  rotation  of  the  earth  on  its  axis, 
founded  upon  the  fact  that  the  pendulum  vibrates 
constantly  in  one  plane.*  (e.)  By  observing  the  dif- 
ference in  the  length  of  a  seconds-pendulum  at  the 

*  A  pendulum  220  feet  in  length  was  suspended  from  the  dome  of  the 
Pantheon  in  Paris.  The  lower  end  of  the  pendulum  traced  its  vibrations 
north  and  south  upon  a  table  beneath,  sprinkled  with  fine  sand.  These 
paths  did  not  coincide,  but  at  each  return  to  the  outside,  the  pendulum 
marked  a  point  to  the  right.  At  the  poles  of  the  earth  the  pendulum, 
constantly  vibrating  in  the  same  vertical  plane,  would  perform  a  complete 
revolution  in  twenty-four  hours,  making  thus  a  kind  of  clock.  At  the  equa- 
tor it  would  not  change  east  or  west,  as  the  plane  of  vibration  would  go 
forward  with  the  diurnal  rotation  of  the  earth.  The  shifting  of  the  plane 
would  increase  as  the  pendulum  was  carried  north  or  south  from  the  equa- 
tor. 


PRACTICAL     QUESTIONS.  78 

top  of  a  mountain  and  at  the  level  of  the^sea,  the 
density  of  the  earth  may  be  estimated. 


PRACTICAL     QUESTIONS. 

1.  When  an  apple  falls  to  the  ground,  does  the  earth  rise  to  meet  it? 

2.  Will  a  body  weigh  more  in  a  valley  than  on  a  mountain  ? 

3.  Will  a  pound  weight  fall  more  slowly  than  a  two-pound  weight? 

4.  How  deep  is  a  well  if  it  takes  three  seconds  for  a  stone  to  fall  to 
the  bottom? 

5.  Is  the  center  of  gravity  always  within  a  body— as,  for  example,  a 
pair  of  tongs? 

6.  In  a  ball  of  equal  density  throughout,  where  is  the  center  of  grav- 
ity? 

7.  Why  does  a  ball  roll  down  hill  ? 

8.  Why  is  it  easier  to  roll  a  round  body  than  a  square  one? 

9.  Why  is  it  easier  to  tip  over  a  load  of  hay  than  one  of  stone? 

10.  Why  is  a  pyramid  such  a  stable  structure? 

11.  When  a  hammer  is  thrown,  on  which  end  does  it  most  often  strike? 

12.  Why  does  a  rope-walker  carry  a  heavy  balancing-pole  ? 

13.  What  would  become  of  a  ball  if  dropped  into  a  hole  bored  through 
the  center  of  the  earth? 

14.  Would  a  clock  lose  or  gain  time  if  carried  to  the  top  of  a  mount- 
ain ?    If  carried  to  the  North  Pole  ? 

15.  In  the  winter,  would  you  raise  or  lower  the  pendulum-bob  of  your 
clock? 

16.  Why  is  the  pendulum-bob  generally  made  flat? 

17.  What  "beats-off"  the  time  in  a  watch? 

18.  What  should  be  the  length  of  a  pendulum  to  vibrate  minutes  at 
the  latitude  of  New  York?    Solution:  (1  sec.)2  :  (60  sec.)2  ::  39.1  in.  :  x  =  2.2 
+  miles. 

19.  What  should  be  the  length  of  the  above  to  vibrate  half -seconds  ? 
Quarter-seconds?    Hours? 

20.  What  is  the  proportionate  time  of  vibration  of  two  pendulums,  re- 
spectively 16  and  64  inches  long? 

21.  Why,  when  you  are  standing  erect  against  a  wall,  and  a  piece  of 
money  is  placed  between  your  feet,  can  you  not  stoop  forward  and  pick  it 
up? 

22.  If  a  tower  were  198  ft.  high,  with  what  velocity  would  a  stone, 
dropped  from  the  summit,  strike  the  ground  ?    (In  these  problems  on  falling 
bodies  we  may  disregard  the  resistance  of  the  air.) 

23.  A  body  falls  in  5  seconds ;  with  what  velocity  does  it  strike  the 
ground? 


74  ATTRACTION. 

24.  How  far  will  a  body  fall  in  10  seconds  ?    "With  what  velocity  will  it 
strike  the  ground? 

25.  A  body  is  thrown  upward  with  a  velocity  of  192  ft.  the  first  second : 
to  what  height  will  it  rise  ? 

26.  A  ball  is  shot  upward  with  a  velocity  of  256  ft. ;  to  what  height 
will  it  rise?    How  long  will  it  continue  to  ascend? 

27.  Why  do  not  drops  of  water,  falling  from  the  clouds,  strike  with  a 
force  equal  to  that  calculated  according  to  the  laws  of  falling  bodies?    Be- 
cause the  mass  of  each  drop  is  so  small  in  proportion  to  its  surface  that  the 
resistance  of  the  air  soon  balances  the  acceleration  of  gravity,  so  that  they 
fall  with  uniform  velocity  instead  of  accelerated  velocity. 

28.  Are  any  two  plumb-lines  parallel  ? 

29.  A  stone  let  fall  from  a  bridge  strikes  the  water  in  3  seconds.    What 
is  the  height  ? 

30.  A  stone  falls  from  a   church-steeple  in  4  seconds.    What  is  the 
height  of  the  steeple? 

31.  How  far  would  a  body  fall  in  the  first  second  at  a  distance  of 
12,000  miles  above  the  earth's  surface? 

32.  A  body  at  the  surface  of  the  earth  weighs  100  tons ;  what  would 
be  its  weight  1,000  miles  above  ? 

33.  A  boy  wishing  to  find  the  height  of  a  steeple,  lets  fly  an  arrow  that 
just  reaches  the  top  and  then  falls  to  the  ground.     It  is  in  the  air  6  sec- 
onds.   Required  the  height. 

34.  An  object  let  fall  from  a  balloon  reaches  the  ground  in  10  seconds. 
Required  the  distance. 

35.  In  what  time  will  a  pendulum  40  ft.  long  make  a  vibration? 

36.  Two  bodies  in  space  are  12  miles  apart.     Their  masses  are,  respect- 
ively, 100  and  200  Ibs.    If  they  should  fall  together  by  their  mutual  attrac- 
tion, what  portion  of  the  distance  would  be  passed  over  by  each  body? 

37.  If  a  body  weighs  2,000  Ibs.  upon  the  surface  of  the  earth,  what 
would  it  weigh  2,000  miles  above?    500  miles  above? 

38.  At  what  distance  above  the  earth  will  a  body  fall,  the  first  second, 
21^  inches? 

39.  How  far  will  a  body  fall  in  8  seconds  ?    In  the  8th  second  ?    In  10 
seconds?    In  the  30th  second? 

40.  How  long  would  it  take  for  a  pendulum  one  mile  in  length  to  make 
a  vibration? 

41.  What  would  be  the  time  of  vibration  of  a  pendulum  64  meters 
long? 

42.  A  ball  is  dropped  from  a  height  of  64  ft.     At  the  same  moment  a 
second  ball  is  thrown  upward  with  sufficient  velocity  to  reach  the  same 
point.    How  far  from  the  ground  will  the  two  balls  pass  each  other? 

43.  Explain  the  following  fact:   A  straight  stick  loaded  with  lead  at 
one  end,  can  be  more  easily  balanced  vertically  on  the  finger  when  the 
loaded  end  is  upward  than  when  it  is  downward. 

44.  If  a  body  weighing  a  pound  on  the  earth  were  carried  to  the  sun  it 
would  weigh  about  27  Ibs.    How  much,  would  it  then  attract  the  sun? 


SUMMARY.  75 

45.  Why  does  watery  vapor  float  and  rain  fall? 

46.  If  a  body  weighs  10  kilos,  on  the  surface  of  the  earth,  what  would 
it  weigh  1,000  kilometers  above  (the  earth's  radius  being  6,366  km.)  ? 

47.  A  body  is  thrown  vertically  upward  with  a  velocity  of  100  meters ; 
how  long  before  it  will  return  to  its  original  position  ? 

48.  Required  the  time  needed  for  a  body  to  fall  a  distance  of  2,000 
meters. 

49.  What  would  be  the  time  of  vibration  of  a  pendulum  39.1  inches 
long  at  the  surface  of  the  moon,  where  the  acceleration  of  gravity  is  only 
4.8  ft.  ? 

50.  What  would  be  the  time  of  vibration  for  the  same  pendulum  at 
the  surface  of  the  sun,  where  the  acceleration  of  gravity  is  27  times  what 
it  is  at  the  earth's  surface? 

51.  How  many  vibrations  per  minute  would  be  made  at  the  surface  of 
the  moon  by  a  pendulum  40  ft.  long  ? 

52.  A  pendulum  vibrates  200  times  in  15  minutes.    What  is  its  length  ? 

53.  For  a  certain  clock  in  New  York  the  pendulum  was  made  500  Ibs. 
in  weight.    What  was  the  object  in  making  it  so  heavy? 

54.  Pendulums  are  often  supported  by  knife-edges  of  steel  resting  on 
plates  of  agate.    Why? 

55.  The  acceleration  of  gravity  at  the  equator  is  32.088  ft. ;  at  the  pole, 
32.253  ft.    If  a  pendulum  vibrates  3,600  times  an  hour  at  the  equator,  how 
many  times  an  hour  will  it  vibrate  at  the  pole  ? 


SUM  MARY. 

THERE  are  certain  forces  operating  between  molecules  and  act- 
ing only  at  insensible  distances,  which  are  known  as  the  Molecular 
Forces.  The  one  which  ties  together  molecules  of  the  same  kind 
is  styled  cohesion.  The  relation  between  this  force  and  that  of 
heat  chiefly  determines  whether  a  body  is  solid,  liquid,  or  gase- 
ous. Under  the  action  of  cohesion,  liquids  tend"*to  form  spheres ; 
and  many  solids,  crystals.  The  processes*  of  welding  and  tem- 
pering, and  the  annealing  of  iron  and  glass,  illustrate  curious 
modifications  of  the  cohesive  force.  Molecules  of  different  kinds 
are  held  together  by  adhesion.  Its  action  is  seen  in  the  use  of 
cement,  paste,  etc.,  in  the  solution  of  solids,  in  capillarity,  diffu- 
sion of  gases,  and  osmose. 

Gravitation,    though   weak,*   compared    with    cohesion,    acts 

*  As  the  attraction  of  gravitation  acts  so  commonly  upon  great  masses 
of  matter,  we  are  apt  to  consider  it  a  tremendous  force.  We,  however, 
readily  detect  its  relative  feebleness  when  we  compare  the  weight  of  bodies 


76  ATTRACTION. 

universally.  Its  force  is  directly  as  the  product  of  the  attract- 
ing and  attracted  masses,  and  inversely  as  the  square  of  their 
distance  apart.  Gravity  makes  a  stone  fall  to  the  ground.  The 
earth  and  a  kilogram  of  iron  in  mid-air  attract  each  other  equally, 
but  the  mass  of  the  former  is  so  much  greater  that  they  move 
toward  each  other  with  unequal  velocity,  and  the  motion  of  the 
earth  is  imperceptible.  "Weight  is  the  measure  of  the  attraction 
of  the  earth.  At  the  center  of  the  earth  the  weight  of  a  body 
would  be  nothing ;  at  the  poles  it  would  be  greatest,  and  at  the 
equator  least.  Increase  of  distance  above  or  far  below  the  sur- 
face of  the  earth  will  diminish  weight.  "Were  the  resistance  of 
the  air  removed,  all  bodies  would  fall  with  equal  rapidity.  The 
laws  of  falling  bodies  may  be  studied  with  the  aid  of  Atwood's 
Machine.  The  first  second  a  body  falls  16  ft.  (4.9  meters),  and 
gains  a  velocity  of  32  ft.  (9.8  meters).  In  general,  the  final  ve- 
locity of  a  falling  body  is  32  ft.,  multiplied  by  the  number  cor- 
responding to  the  second,  and  the  distance  is  16  ft.  multiplied 
by  tha  square  of  the  number  expressing  the  seconds.  The  center 
of  gravity  is  the  point  about  which  the  weights  of  all  the  par- 
ticles composing  a  body  will  balance  one  another,  i.  e,,  be  in 
equilibrium.  There  are  three  states  of  equilibrium — stable,  un- 
stable, and  indifferent — according  as  the  point  of  support  in  a 
body  is  above,  below,  or  at  the  center  of  gravity.  As  the  center 
of  gravity  tends  to  seek  the  lowest  point,  its  position  determines 
the  stability  of  a  body.  A  body  suspended  so  as  to  swing  freely 
is  a  pendulum.  The  time  of  a  pendulum's  vibration  is  independ- 
ent of  its  material,  proportional  to  the  square  root  of  its  length 
and  variable  according  to  the  latitude.  The  pendulum  is  our 
time-keeper  and  "useful  in  many  scientific  investigations. 

"We  are  so  accustomed  to  see  all  the  objects  around  us  pos- 
sess weight,  that  we  can  hardly  conceive  of  a  body  deprived  of 
a  property  which  we  are  apt  to  consider  as  an  essential  attri- 
bute of  matter.  Nothing  is  more  natural,  apparently,  than  the 
falling  of  a  stone  to  the  ground.  "Yet,"  says  D'Alembert,  "it 
is  not  without  reason  that  philosophers  are  astonished  to  see  a 
stone  fall,  and  those  who  laugh  at  their  astonishment  would 


with  their  tenacity.— Example :  Think  how  much  easier  it  is  to  lift  an  iron 
wire  against  gravity  than  to  pull  it  to  pieces  against  cohesion. 


HISTORICAL     SKETCH.  77 

soon  share  it  themselves,  if  they  would  reflect  on  the  subject/' 
Gravity  is  constantly  at  work  about  us,  at  one  moment  produc- 
ing equilibrium  or  rest,  and  at  another,  motion.  When  it  seems 
to  be  destroyed,  it  is  only  counterbalanced  for  a  time,  and  re- 
mains, apparently,  as  indestructible  as  matter  itself.  The  sta- 
bility and  the  incessant  changes  of  nature  are  alike  due  to  its 
action.  Not  only  do  rivers  flow,  snows  fall,  tides  rise,  and 
mountains  stand  in  obedience  to  gravitation,  but  smoke  ascends 
and  clouds  float  through  the  combined  influence  of  heat  and 
weight. 


HISTORICAL     SKETCH. 

THE  latter  part  of  the  sixteenth  century  witnessed  the  estab- 
lishment of  the  principles  of  falling  bodies.  Galileo,  while  sitting 
in  the  cathedral  at  Pisa  and  watching  the  swinging  of  an  im- 
mense chandelier  which  hung  from  its  lofty  ceiling,  noticed  that 
its  vibrations  were  isochronous.  This  was  the  germ-thought  of 
the  pendulum  and  the  clock.  Up  to  his  time  it  had  been  taught 
that  a  4-lb.  weight  would  fall  twice  as  fast  as  a  2-lb.  one.  He 
proved  the  fallacy  of  this  view  by  dropping  from  the  Leaning 
Tower  of  Pisa  balls  of  different  metals— gold,  copper,  and  lead. 
They  all  reached  the  ground  at  nearly  the  same  moment.  The 
slight  variation  he  correctly  accounted  for  by  the  resistance  of 
the  air,  which  was  not  the  same  for  all. 

Newton  and  his  immediate  predecessors  knew  the  law  of 
terrestrial  gravity  as  manifested  in  falling  bodies.  When  quite 
a  young  man,  Newton  entertained  the  idea  that  the  attraction 
which  draws  bodies  downward  at  the  earth's  surface  must  exist 
also  between  masses  widely  separated  in  sfcace,  such  as  the  earth 
and  the  moon.  To  test  this,  he  calculated  how  far  the  moon 
bends  from  a  straight  line,  i.  e.,  falls  toward  the  earth  every 
second.  Knowing  the  distance  a  body  falls  in  a  second  at  the 
surface  of  the  earth,  he  endeavored  to  see  how  far  it  would  fall 
at  the  distance  of  the  moon.  For  years  he  toiled  over  this  prob- 
lem, but  an  erroneous  estimate  of  the  earth's  diameter  then 
accepted  by  physicists  prevented  his  obtaining  a  correct  result. 
Finally,  a  more  accurate  measurement  having  been  made,  he 
inserted  this  in  his  calculations.  Finding  the  result  was  likely 


78  ATTRACTION. 

to  verify  his  conjecture,  his  hand  faltered  with  the  excitement, 
and  he  was  forced  to  ask  a  friend  to  complete  the  task.  The 
truth  was  reached  at  last,  and  the  grand  law  of  gravitation 
discovered  (1682). 

The  sun-dial  was  doubtless  the  earliest  device  for  keeping 
time.  The  clepsydra  was  afterward  employed.  This  consisted 
of  a  vessel  containing  water,  which  slowly  escaped  into  a  dish 
below,  in  which  was  a  float  that  by  its  height  indicated  the 
lapse  of  time.  King  Alfred  used  candles  of  a  uniform  size,  six 
of  which  lasted  a  day.  The  first  clock  erected  in  England,  about 
1288,  was  considered  of  so  much  importance  that  a  high  official 
was  appointed  to  take  charge  of  it.  The  clocks  of  the  middle 
ages  were  extremely  elaborate.  They  indicated  the  motions  of 
the  heavenly  bodies ;  birds  came  out  and  sang  songs,  cocks  crowed, 
and  trumpeters  blew  their  horns ;  chimes  of  bells  were  sounded, 
and  processions  of  dignitaries  and  military  officers,  in  fan- 
tastic dress,  marched  in  front  of  the  dial  and  gravely  announced 
the  time  of  day.  Watches  were  made  at  Nuremberg  in  the 
fifteenth  century.  They  were  styled  Nuremberg  eggs.  Many 
were  as  small  as  the  watches  of  the  present  day,  while  others 
were  as  large  as  a  dessert-plate.  They  had  no  minute  or  second 
hand,  and  required  winding  twice  per  day. 

On  Attraction,  as  well  as  on  subsequent  topics  treated  in  this 
book,  consult  Guillemin's  "Forces  of  Nature;"  Atkinson's  "  Ga- 
not's  Physics";  Arnott's  "Elements  of  Physics";  Snell's  "  Olm- 
stead's  Natural  Philosophy";  Stewart's  "Elementary  Physics"; 
Silliman's  "Physics";  Everett's  "Text-book  of  Physics";  Young's 
"Lectures  on  Natural  Philosophy";  "Appleton's  Cyclopedia," 
articles  on  Clocks  and  "Watches,  Weights  and  Measures,  Gravi- 
tation, Mechanics,  etc.;  Peck's  "  Ganot's  Natural  Philosophy"; 
Miller's  "Chemical  Physics,"  Chap.  III.,  on  Molecular  Force; 
Weinhold's  "  Experimental  Physics";  Pickering's  "  Elementary 
Physical  Manipulation";  "Fourteen  Weeks  in  Astronomy,"  sec- 
tions on  Galileo  and  Newton,  pp.  29-34. 

The  current  numbers  of  "  Harper's  Magazine,"  "The  Century 
Magazine,"  "Scribner's  Magazine,"  "Popular  Science  Monthly," 
"Boston  Journal  of  Chemistry,"  "Scientific  American,"  "  Knowl- 
edge," and  "Nature,"  contain  the  latest  phases  of  science. 


ELEMENTS  OF  MACHINES. 


NATURE  is  a  reservoir  of  power.  Tremendous  forces  are  all  about  us, 
but  they  are  not  adapted  to  our  use.  We  need  to  remold  the  energy  to 
fit  our  wants.  A  water-fall  can  not  grind  corn  nor  the  wind  draw  water. 
Yet  a  machine  will  gather  up  these  wasted  forces,  and  turn  a  grist-mill  or 
work  a  pump.  A  kettle  of  boiling  water  has  little  of  promise ;  but  hus- 
band its  energy  in  the  steam-engine,  and  it  will  weave  cloth,  forge  an 
anchor,  or  bear  our  burdens  along  the  iron  track. 

"  The  hero  in  the  fairy  tale  had  a  servant  who  could  eat  granite  rocks, 
another  who  could  hear  the  grass  grow,  and  a  third  who  could  run  a  hun- 
dred leagues  in  half  an  hour.  So  man  in  nature  is  surrounded  by  a  gang 
of  friendly  giants  who  can  accept  harder  stints  than  these.  There  is  no 
porter  like  gravitation,  who  will  bring  down  any  weight  you  can  not  carry, 
and  if  he  wants  aid,  knows  how  to  get  it  from  his  fellow-laborers.  "Water 
sets  his  irresistible  shoulder  to  your  mill,  or  to  your  ship,  or  transports  vast 
bowlders  of  rock,  neatly  packed  in  his  iceberg,  a  thousand  miles." 


ANALYSIS  OF  THE  ELEMENTS  OF  MACHINES. 


—  THE  SIMPLE  MACHINES. 

—  THE  LAW  OF  MECHANICS. 


1.  THE  LEVER. 


1.  Definition. 


2.  Three  Classes  f 
of  Levers. 


2.  THE    WHEEL 
AXLE. 


AND 


3.  THE  INCLINED  PLANE. 


4.  THE  SCREW. 


5.  THE  WEDGE. 


Class' 
Second  Class. 

(3.)  Third  Class. 

3.  Law  of  Equilibrium. 

4.  Steelyard. 

5.  Compound  Lever. 

1.  Definition  and  Illustration. 

2.  Law  of  Equilibrium. 

3.  Wheel-work. 

1.  Definition  and  Illustration, 

2.  Law  of  Equilibrium. 

j  1.  Definition  and  Illustration. 
(  2.  Law  of  Equilibrium. 

j  1.  Definition  and  Illustration- 
1  2.  Law  of  Equilibrium. 


6.  THE  PULLEY. 


1.  Definition  and  Illustration. 

2.  Fixed  and  Movable  Pulleys. 

3.  Combinations  of  Pulleys. 

4.  Law  of  Equilibrium. 

7.  CUMULATIVE  CONTRIVANCES. 

8.  PERPETUAL  MOTION. 


ELEMENTS  OF  MACHINES. 

The  Simple  Machines  are  the  elements  to  which 
all  machinery  can  be  reduced.  The  watch  with  its 
complex  system  of  wheel-work,  and  the  engine  with 
its  belts,  cranks,  and  pistons,  are  only  various  modi- 
fications of  some  of  the  six  elementary  forms — the 
lever,  the  wheel  and  axle,  the  inclined  plane,  the 
screw,  the  wedge,  and  the  pulley.  These  six  may  be 
still  further  reduced  to  two  —  the  lever  and  the  in- 
clined plane. 

They  are  often  termed  the  Mechanical  Powers, 
but  they  do  not  produce  work ;  they  are  only  the 
means  of  applying  it.  Here  again  the  doctrine  of 
the  Conservation  of  Energy  holds  good.  The  work 
done  by  the  power  is  always  equal  to  the  resistance 
overcome  in  the  weight. 

The  Law  of  Mechanics  is,  the  power  multiplied 
by  the  distance  through  which  it  moves,  is  equal  •  to 
the  weight  multiplied  ~by  the  distance  through  which 
it  moves. — Example:  1  Ib.  of  power  moving  through 
10  feet=  10  Ibs.  of  weight  moving  through  one  foot, 
or  vice  versa.  In  theory,  the  parts  of  a  machine 
have  no  weight,  move  with  no  friction,  and  meet  no 
resistance  from  the  air.  In  practice,  these  influences 
must  be  considered. 


82 


ELEMENTS     OF     MACHINES. 


1.  The  Lever  is  a  bar  turning  on  a  pivot.  The 
force  used  is  termed  the  power  (P),  the  object  to  be 
lifted  the  weight  ("FT),  the  pivot  on  which  the  lever 
turns  the  fulcrum  (F),  and  the  parts  of  the  lever 
each  s'ide  of  the  fulcrum  the  arms. 

THREE  CLASSES  OF  LEVERS.  —  In  the  three  kinds,  the 
fulcrum,  weight,  and  power  are  each  respectively  be- 


FIG.  37. 


FIG.  38. 


fia. 


J7T                                            1 

1 
F 

T 

1       ' 

P                                          , 

V 

7   '                        ' 

A 

J 

First  Class. 


Second  Class. 


Third  Class. 


FIG.  40. 


tween  the  other  two,  as  may  be  seen  by  comparing 
Figs.  37-39. 

First  Class.  —  We  wish  to  lift  a  heavy  stone.  Ac- 
cordingly we  put  one  end  of  a  handspike  under  it, 
and  resting  the  bar  on  a  block  at  I1,  bear  down  at 
P.  —  A  pump-handle  is  a  lever  of 
the  first  class.  The  hand  is  the 
P,  the  water  lifted  the  W,  and 
the  pivot  the  F.  —  A  pair  of  scis- 
sors is  a  double  lever  of  the  same 
class.  The  cloth  to  be  cut  is  the 
W,  the  hand  the  P,  and  the  rivet  the  F. 

Second  Class.  —  We  may  also  raise  the  stone  by 
resting  one  end  of  the  lever  on  the  ground,  which 
acts  as  a  fulcrum,  and  lifting  up  on  the  bar.  —  An 
oar  is  a  lever  of  the  second  class.  The  hand  is  the 


w 


Lifting  a  Stone. 


THE    LEVEE.  83 

P,  the  boat  the    W,  and  the  water  the  F.     In  this 
case  the  F  is  not  immovable. 

Third  Class. — The  treadle  of  a  sewing-machine  is 
a  lever  of  the  third  class.  The  front  end  resting  on 
the  ground  is  the  F,  the  foot  is  the  P,  and  the  force 
is  transmitted  by  a  rod  to  the  W,  the  arm  above. 

LAW  OF  EQUILIBRIUM. — The  product  of  P  multiplied 
by  the  perpendicular  distance  between  its  line  of  ac- 
tion and  F,  is  called  the  mo-  n»4r 
ment  of  P.  In  the  lever,  P 
balances  TFwhen  the  moments 
about  the  fulcrum  are  equal. 
In  Fig.  41,  assume  AB  to  be 
the  initial  position  of  a  lever, 
which  is  then  turned  into  the 
position  A'B'  by  application  of 
the  power,  P,  which  balances  the  weight  W,  its  line 
of  action  being  A'P,  while  that  of  W  is  B'W.  The 
power  moves  through  a  distance  equal  to  AA,  while 
the  weight  moves  through  a  distance  equal  to  BB'. 
But  these  distances  are  proportional  to  A'F  and  B'F. 
We  may  represent  A'F  by  Pd,  the  distance  of  the 
power's'  line  of  action  from  F ;  and  B'F  by  Wdf,  the 
distance  of  the  weight's  line  of  action  from  F.  Sub- 
stituting these  terms  in  the  general  expression  of 
the  law,  we  have, 

W  v  "WV7' 
P  x  Pd  =W  x  Wd'  (1)     P  :  W  :  :  Wd>  :  Pd  (2)       P  =     pd (3) 

In  the  first  and  second  classes,  as  ordinarily  used, 
we  gain  power  and  lose  time  ;  in  the  third  class  we 
lose  power  and  gain  time. 


84 


ELEMENTS     OF     MACHINES. 


The  BALANCE  is  a  iever  of  the  first  class  with 
equal  arms.  The  bar,  AS  (Fig.  42),  has  a  pair  of 
scale  pans  suspended  from  its  ends.  At  the  middle 


PIG.  42. 


Tlie  Balance. 


an  axis,  n,  made  of  steel  and  provided  with  a  knife 
edge,  rests  upon  a  hard  surface,  so  that  the  friction 
may  be  the  least  possible ;  this  is  the  fulcrum. 

The  STEELYARD  (Fig.  43)  is  a  lever  of  the  first  class. 
The  P  is  at  E,  the  F  at  (7,  and  the  TFat  D.    If  the  dis- 


THE     LEVER. 


85 


FIG.  43. 


tance  from  the  pivot  of  the  hook  D  to  the  pivot  of 
the  hook  C  be  one  inch,   and   from   the 
pivot  of  the  hook  Q  to  the  notch  where 
E  hangs  be  1 2  inches,  then  1  Ib.  at  E  will 
balance  12  Ibs.  at  W.    If  the  steelyard  be 
reversed   (Fig.   44),  then  the  distance  of 
the  F  from  the  W  is 
only 


FIG.  44. 


J-  as  great,  and 
1  Ib.  at  E  will  balance 
481bs.atJD.  Two  sets 

of  notches  on  opposite  sides  of  the  bar  cor- 
respond to  these  different  positions. 

The  COMPOUND  LEVER  consists  of  several 
levers  so   connected  that  the  short  arm  of  the  first 
acts  on  the  long  arm  of  the  second,  and  so 
on  to  the  last  of  the  series.    If  the  distance 
of  A   (Fig.  45)   from  the  F  be  four  times 
that  of  B,  a  P  of  5  Ibs.  at  A  will  balance 
a  W  of  20   Ibs.  at  B. 
If    the    arms    of    the 
second    lever    are    of 
the  same  comparative 

length,  a  P  of  20  Ibs.  at  C  will  balance  80 
Ibs.  at  E.  In  the  third  lever,  a  P  df  80  Ibs. 
at  D  will  balance  320  Ibs.  at  G.  With  this 

system  of  three  levers,  5 
Ibs.  at  A  will  according- 
ly balance  320  Ibs.  at  G. 
To  raise  the  W 1  ft.,  how- 

Tbe  Compound  Lever.  ,  •,          -^ 

ever,  the  P  must  move 
64  ft.    Thus  what  is  gained  in  power  is  lost  in  time. 


Fio.  45. 


86  ELEMENTS     OF     MACHINES. 

There  is  no  creation  of  force  by  the  use  of  the 
levers;  on  the  contrary,  there  is  an  appreciable  loss 
because  of  friction. 

Hay  scales  are  constructed  upon  the  principle  of 
the  compound  lever.     Considering  the  large  mass  on 

the  platform  as  the  power, 
its  pressure  is  transmitted 
at  the  points  P  and  P'  (Fig. 
46)  to  a  pair  of  levers  of  the 
third  class,  whose  fulcrums 
are  at  F  and  F'.  Pressure 
is  thus  produced  at  P"  on 
Hay  scales.  another  lever  whose  fulcrum 

is  at  F".  At  the  remote  end  of  this  in  turn,  press- 
ure is  transmitted  by  the  upright  bar  to  the  end,  P"', 
of  a  lever  of  the  first  class  whose  fulcrum  is  at  F'". 
The  weight,  W,  can  be  adjusted  at  will  until  a  bal- 
ance is  secured. 

2.  The  Wheel  and  Axle  is  a 
kind  of  perpetual  lever.  As  both 
arms  work  continuously,  we  are  not 
obliged  to  prop  up  the  W  and  re-ad- 
just the  lever.  In  the  windlass  used 
for  drawing  water  from  a  well,  the 
P  is  applied  at  the  handle,  the  W  is 
the  bucket,  and  the  F  is  the  axis  of  the  windlass. 
The  long  arm  of  the  lever  is  the  length  of  the  handle, 
and  the  short  arm  is  the  semi-diameter  of  the  axle. 
This  is  shown  in  a  cross-section  (Fig.  47)  where  the 
center,  0,  is  the  F,  OA  the  long  arm,  and  OB  the  short 
arm. — In  Fig.  48,  instead  of  turning  a  handle  we  take 


THE     WHEEL     AND     AXLE. 


87 


hold  of  pins  inserted  in  the  rim  of  the  wheel. — Fig. 
4  9  represents  a  capstan  used  FIG.  48. 

on  vessels  for  raising  the  an- 
chor. The  P  is  applied  by 
handspikes  inserted  in  the 
axle. — Fig.  50  shows  a  form 
of  the  capstan  employed  in 
moving  buildings,  in  which  a 
horse  furnishes  the  power. 

LAW  OF  EQUILIBRIUM.  —  By 
turning  the  handle  or  wheel 

around  once,  the  rope  will  be 
wound  around  the  axle  and  the 
W  be  lifted  that  distance.  Ap- 
plying the  law  of  mechanics, 
P  x  the  circumference  of  the 
wheel  =  W  x  circumference  of 
the  axle ;  or,  as  circles  are  pro- 
portional to  their  radii, 


Wheel  and  Axle. 


FIG.  49. 


Capstan. 


P :  W : :  radius  of  the  axle  :  radius  of  the  wheel. 


(4) 


FIG.  50. 


Capstan. 


88 


ELEMENTS     OF     MACHINES. 


51 


. 


w 


WHEEL-WORK  consists  of  a  series  of  wheels  and 
axles  which  act  upon  one  another  on  the  principle  of 
the  compound  lever.  The  projections  on  the  circum- 
ference of  the  wheel  are  termed 
teeth,  on  the  axle  leaves,  and  the 
axle  itself  is  called  a  pinion.  If 
the  radius  of  the  wheel  F  be  12 
.p  inches,  and  that  of  each  pinion  2 
inches,  then  a  P  of  1  Ib.  will  ap- 
ply a  force  of  6  Ibs.  to  the  second 
wheel  E.  If  the  radius  of  this  be  12  inches,  then 
the  second  wheel  will  apply  a  P  of  36  Ibs.  to  the 
third  wheel,  which,  acting  on  its  axle,  will  balance 
a  W  of  2 1 6  Ibs.  The  W  will  pass  through  only  ^ 
the  distance  of  the  P.  We  thus  gain  power  and  lose 
speed.  If  we  wish  to  reverse  this  we  can  apply  the 
P  to  the  axle,  and  so  gain  speed.  This  is  the  plan 
adopted  in  factories,  where  a  water-wheel  furnishes 
abundant  power,  and  spindles  or  other  machines  are 
to  be  turned  with  great  rapidity. 

3.  The  Inclined  Plane. — If  we  wish  to  lift  a 
heavy  cask  into  a  wagon,  we  rest  one  end  of  a  plank 
on  the  wagon-box  and  the  other  on  the  ground.  We 
can  then  easily  roll  the  cask  up  this  inclined  plane. 
When  roads  are  to  be  made  over  steep  hills,  they 
are  sometimes  constructed  around  the  hill,  like  the 
thread  of  a  screw,  or  in  a  winding  manner  as  shown 
in  Fig.  52.  There  is  a  remarkable  ascent  of  this  kind 
on  Mount  Royal,  Montreal. 

LAW  OF  EQUILIBRIUM. — In  Fig.  53  the  P  must  de- 
scend vertically  a  distance  equal  to  the  length  of  the 


THE     INCLINED     PLANE. 


89 


plane,  AC,  in  order  to  move  the  W  from  A  to  C  and" 
thus  elevate  it  through  the  vertical  height  BC.    Ap- 


Fio 


Inclined  Plane. 


plying  the  law  of  mechanics,  P  x  length  of  inclined 

plane  =  W  x  height  of  inclined  plane ;  hence, 

P  :  W  : :  height  of  inclined  plane  :  length  of  inclined  plane.*  (5) 


100   ft.,  then  a  horse 

FIG.  53. 

W 


Inclined  Plane. 


If  a  road  ascend  1  ft.  in 
drawing  up  a  wagon  has  to 
lift  only  yj^  of  the  load,  besides 
overcoming  the  friction.  A 
body  sliding  down  a  perfectly 
smooth  inclined  plane  acquires 
the  same  velocity  that  it  would  in  falling  the  same 

*  If  we  roll  into  a  wagon  a  barrel  of  pork,  weighing  200  Ibs.,  up  a  plane 
12  ft.  long  and  3  ft.  high,  we  have  12  ft.  :  3  ft.  : :  200  Ibs.  :  x  =  50  Ibs.  We 
lift  only  50  Ibs.,  or  i  of  the  barrel,  but  we  move  it  through  four  times  the 
space  that  would  be  necessary  if  we  could  elevate  it  directly  into  the  wagon. 
We  thus  lose  speed  and  gain  power.  The  longer  the  inclined  plane,  the 
heavier  the  load  we  can  lift,  but  the  more  time  it  will  take  to  do  it. 


90- 


ELEMENTS     OF     MACHINES. 


FIG.  54. 


height  perpendicularly.  A  train  descending  a  grade 
of  1  ft.  in  100  ft.  tends  to  go  down  with  a  force 
equal  to  ^  of  its  weight.* 

4.  The  Screw  consists  of  an  inclined  plane  wound 
around  a  cylinder,  the  former  being  called  the  thread, 
and  the  latter  the  body.  It  works  in  a  nut  which 
is  fitted  with  reverse  threads  to  move  on  the  thread 
of  the  screw.  The  nut  may  turn  on  the  screw,  or  the 
screw  in  the  nut.  The  P  may  be  applied  to  either, 

by  means  of  a  wrench 
or  lever.  The  screw  is 
used  in  vises ;  in  raising 
buildings ;  in  copying  let- 
ters, and  in  presses  for 
squeezing  the  juice  from 
apples,  sugar-cane,  etc. 

LAW  OF  EQUILIBRIUM. — 
When  the  P  is  applied  at 
the  end  of  a  lever,  at- 
tached to  the  head  of  the 
screw,  it  describes  a  circle 
screw.  of  which  the  lever  is  the 

radius.  The  distance  through  which  the  P  passes,  is 
the  circumference  of  this  circle  ;  and  the  height  to 
which  the  W  is  elevated  at  each  revolution  of  the 
screw,  is  the  distance  between  two  of  the  threads. 

*  Near  Lake  Lucerne  is  a  forest  of  firs  on  the  top  of  Mount  Pilatus,  an 
almost  inaccessible  Alpine  summit.  By  means  of  a  wooden  trough,  the 
log  is  conducted  into  the  water  below,  a  distance  of  eight  miles,  in  but 
little  more  than  as  many  minutes.  The  force  with  which  it  falls  is  so 
prodigious,  that  if  it  jumps  out  of  the  trough  it  is  dashed  to  pieces. 


THE     WEDGE     AND     PULLEY. 


91 


Applying  the  law  of  mechanics,  P  x  circumference 
of  circle  =  W  x  interval  between  the  threads ;  hence, 

P  :  W  : :  interval  \ :  circumference (6) 

The  efficiency  of  the  screw  may  be  increased  by 
lengthening  the  lever,  or  by  diminishing  the  distance 
between  the  threads. 

5.  The  Wedge  consists  generally 
of  two  inclined  planes  placed  back  to 
back.  It  is  used  for  splitting  wood 
and  stone  and  lifting  vessels  in  the 
dock.  Leaning  chimneys  have  been 
righted  by  wedges  driven  in  on  the 
lower  side.  Nails,  needles,  pins,  knives, 
axes,  etc.,  are  made  on  the  principle  of  the  wedge. 

THE  LAW  OF  EQUILIBRIUM  is  the  same  as  that  of 
inclined  plane — viz.:* 

P  :  W  : :  thickness  of  wedge  :  length  of  wedge.  .  .  (7) 


Wedge. 


Pis.  56. 


W 


Fixed  Pulley. 


6.  The  Pulley  consists  of  a  wheel, 
within  the  grooved  edge  of  which  runs  a 
cord. 

A  FIXED  PULLEY  (Fig.  56)  merely 
changes  the  direction  of  the  force.  There 
is  no  gain  of  power  or  speed,  as  the  hand 
P  must  move  down  as  much  as  the 
weight  W  rises,  and  both  with  the  same 


*  In  practice,  however,  this  by  no  means  accounts  for  the  prodigious 
power  of  the  wedge.  Friction,  in  the  other  mechanical  powers,  diminishes 
their  efficiency ;  in  this  it  is  essential,  else  the  wedge  would  fly  back  and 
the  effect  be  lost.  In  the  others,  the  P  is  applied  as  a  steady  pressure ;  in 
this  it  is  a  sudden  blow,  and  depends  upon  the  kinetic  energy  expended  in 
the  stroke  of  the  hammer. 


92 


ELEMENTS     OF     MACHINES. 


velocity.    It  is  simply  a  lever  of  the  first  class  with 
FIG.  sr.       equal  arms.    By  its  use   a   man   standing 
on  the  ground  will  hoist  a  flag  to  the  top 
of  a  lofty  pole,  and  thus  avoid  the     pIG.  5s. 
trouble  and  danger  of  climbing  up 
with    it.      Two    fixed   pulleys,    ar- 
ranged as  shown  in  Fig.  57,  make 
it  -possible  to  elevate  a  heavy  load 
to  the   upper  story  of 
a    building    by    horse- 
power. 

Application  of  Fixed  Pulleys.  A   MOVABLE   PULLEY 

is  represented  in  Fig.  58.  One  half  of  the  barrel  is 
sustained  by  the  hook  while  the  hand  lifts  the  other. 
As  the  P  is  one  half  the  W,  it  must  move  through 
twice  the  space  ;  in  other  words,  by  taking  twice  the 
time  we  can  lift  twice  as  much.  Thus  power  is 

gained  and  time  lost. 

We  may  also   explain 
p     the  single  movable  pulley 

by  Fig.  59.    A  represents 

the  F,  R  the  W  acting  in 

the  line   OR,  and  B  the 

P  acting  in  the  line  BP. 

This  is  a  lever  of  the  sec- 

ond class  ;    and  as  A  0  =   P 


Systems  of  Pulleys. 


FIG. 


FIG.  Cl. 


COMBINATIONS  OF  PULLEYS.  —  (1.) 
In  Fig.  60,  we  have  the  W  sus- 
tained by  three  cords,  each  of  which 
is  stretched  by  a  tension  equal  to  the  P;  hence, 


THE     PULLEY. 


93 


FIG.  62, 


FIG.  63. 


1  lb.  of  power  will  balance  3  Ibs.  of  weight.  (2.)  In 
Fig.  61,  the  P  will  sustain  a  W  of  4  Ibs.  (3.)  In 
Fig.  62,  the  cord  marked  1  1  has  a  tension  equal 
to  P  in  each  part ;  the  one 
marked  2  2  has  a  tension 
equal  to  2P  in  each  part, 
and  so  on  with  the  others. 
The  total  tension  acting  on 
W  is  16;  hence,  W=16P. 
In  this  system,  D  rises  twice 
as  fast  as  (7,  four  times  as 
fast  as  .B,  etc.  Work  must 
stop  when  D  reaches  E,  which 
gives  little  sweep  to  A  for 
lifting  W.  (4.)  Fig.  63  rep- 


w 


System  of 
Pulleys. 


Tackle  Block. 


resents  the   ordinary  "  tackle-block "  used 
by  mechanics. 

LAW  OF  EQUILIBRIUM. — When  a  continuous  rope  is 
used,  let  n  represent  the  number  of  separate  parts 
of  the  cord  which  sustain  the  movable  block.  We 
then  have 

j>  =  £. ... ... . : . . .  (8) 

When  the  number  of  movable  arid  of  fixed  pulleys 
is  equal,  in  general,  W  —  P  x  twice  the  number  of 
movable  pulleys. 

7.  Cumulative  Contrivances.  —  A  hammer,  club, 
pile-driver,  sling,  fly-wheel,  etc.,  are  instruments  for 
accumulating  energy  to  be  used  at  the  proper  mo- 
ment. Thus  we  may  press  a  hammer  on  the  head 
of  a  nail  with  all  our  strength  to  no  purpose ;  but 


94  ELEMENTS     OF     MACHINES. 

swing  the  hammer  the  length  of  the  arm,  and  the 
blow  will  bury  the  nail  to  the  head.  The  strength 
of  our  muscles  and  the  attraction  of  gravity  during 
the  fall  both  gather  energy  to  be  exerted  at  the  in- 
stant of  contact.  A  fly-wheel  by  its  momentum 
equalizes  an  irregular  force,  or  produces  a  sudden 
effect.* 

8»  Perpetual  Motion. — It  is  impossible  to  make  a 
machine  capable  of  perpetual  motion.  No  combina- 
tion can  produce  energy ;  it  can  only  direct  that 
which  is  applied.  In  all  machinery  there  is  friction ; 
this  must  ultimately  exhaust  the  power  and  bring 
the  motion  to  rest.  The  only  question  is,  how  long 
a  time  will  be  required  for  the  leakage  to  drain  the 
reservoir.  Every  year  brings  to  light  new  seekers 
after  perpetual  motion.  The  mere  fact  that  a  man 
devotes  himself  to  the  solution  of  this  impossible 
problem  is  now  generally  regarded  as  a  proof  that 
either  his  mental  balance  has  been  disturbed,  or  his 
knowledge  of  the  laws  of  nature  is  too  meager  to 
entitle  him  to  consideration. 


PRACTICAL     QUESTIONS. 

1.  Describe  the  rudder  of  a  boat  as  a  lever.  A  door.  A  door-latch.  A 
lemon-squeezer.  A  pitchfork.  A  spade.  A  shovel.  A  sheep-shears.  A 
poker.  A  pair  of  tongs.  A  balance.  A  pair  of  pincers.  A  wheelbarrow. 
A  man  pushing  open  a  gate  with  his  hand  near  the  hinge.  A  sledge-ham- 


*  We  see  the  former  illustrated  in  a  sewing-machine,  and  the  latter 
in  a  punch  operated  by  a  treadle.  In  the  one  case,  the  irregular  action  of 
the  foot  is  turned  into  a  uniform  motion,  and  in  the  other  it  is  concen- 
trated in  a  heavy  blow  that  will  pierce  a  thick  piece  of  metal. 


PRACTICAL     QUESTIONS.  95 

mer,  when  the  arm  is  swung  from  the  shoulder.     A  nut-cracker.    The  arm 
(see  "Physiology,'   p.  48). 

2.  Show  the  change  that  occurs  from  the  second  to  tl^e  third  class  of 
lever,  when  you  take  hold  of  a  ladder  at  one  end  and  raise  it  against  a 
building. 

3.  Why  is  a  pinch  from  the  tongs  near  the  hinge  more  severe  than  one 
near  the  end? 

4.  Two  persons  are  carrying  a  weight  of  250  Ibs.,  hanging  between  them 
from  a  pole  10  ft.  in  length.     Where  should  it  be  suspended  so  that  one 
will  lift  only  50  Ibs.  ? 

5.  In  a  lever  of  the  first  class,  6  ft.  long,  where  should  the  F  be  placed 
so  that  a  P  of  1  Ib.  will  balance  a  W  of  23  Ibs.  ? 

6.  What  P  would  be  required  to  lift  a  barrel  of  pork  with  a  windlass 
whose  axle  is  1  ft.  in  diameter,  and  handle  3  ft.  long? 

7.  What  sized  axle,  with  a  wheel  6  ft.  in  diameter,  would  be  required 
to  balance  a  W  of  one  ton  by  a  P  of  100  Ibs.  ? 

8.  What  number  of  movable  pulleys  would  be  required  to  lift  a  W  of 
200  Ibs.  by  a  P  of  25  Ibs.  ? 

9..  How  many  pounds  could  be  lifted  with  a  system  of  4  movable  pul- 
leys by  a  P  of  100  Ibs.  ? 

10.  What  W  could  be  lifted  with  a  single  horse-power  *  acting  on  a  sys- 
tem of  pulleys  shown  in  Fig.  62? 

11.  What  distance  should  there  be  between  the  threads  of  a  screw  to 
enable  a  P  of  25  Ibs.  acting  on  a  handle  3  ft.  long,  to  lift  a  ton  ? 

12.  How  high  would  a  P  of  12  Ibs.,  moving  16  ft.  along  an  inclined 
plane,  lift  a  W  of  96  Ibs.  ? 

13.  I  wish  to  roll  a  barrel  of  flour  into  a  wagon,  the  box  of  which  is 
4  ft.  from  the  ground.     I  can  lift  but  24  Ibs.     How  long  a  plank  must 
I  get? 

14.  What  W  can  be  lifted  with  a  P  of  100  Ibs.  acting  on  a  screw  hav- 
ing threads  1  in.  apart,  and  a  handle  4  ft.  long  ? 

15.  What  is  the  object  of  the  balls  often  cast  on  the  ends  of  the  han- 
dle of  the  screw  used  in  presses  for  copying  letters? 

16.  In  a  steelyard  2  ft.  long,  the  distance  from  the  weight-hook  to  the 
fulcrum-hook  is  2  in. ;  how  heavy  a  body  can  be  weighed  with  a  pound 
weight  ? 

17.  Describe  the  change  from  the, first  to  the  third  class  of  lever,  in 
the  different  ways  of  using  a  pitchfork  or  a  spade. 

18.  Why  are  not  blacksmiths'  tongs  and  fire-tongs  constructed  on  the 
same  principle? 

19.  In  a  lever  of  the  third  class,  what  W  will  a  P  of  50  Ibs.  balance,  if 
one  arm  be  12  ft.  and  the  other  3  ft.  long? 


*  A  horse-power  is  a  force  which  is  equivalent  to  550  foot-pounds,  i.  e.,  can 
raise  against  gravity  550  Ibs.  one  foot  in  one  second,  or  33,000  Ibs.  one  foot 
in  one  minute. 


96  ELEMENTS     OF     MACHINES. 

20.  In  a  lever  of  the  second  class,  what  W  will  a  P  of  50  Ibs.  balance, 
with  a  lever  12  ft.  long,  and  the  W  3  ft.  from  the  F? 

21.  In  a  lever  of  the  first  class,  what  W  will  a  P  of  50  Ibs.  balance, 
with  a  lever  12^.  long,  and  the  F  3  ft.  from  the  W? 

22.  In  a  wheel  and  axle,  the  P  =  40  Ibs.,  the  W  =  360  Ibs.,  and  the 
diameter  of  the  axle  =  8  in.    Required  the  circumference  of  the  wheel. 

23.  Suppose,  in  a  wheel  and  axle,  the  P  =  20  Ibs.,  the  W  =  240  Ibs.,  and 
the  diameter  of  the  wheel  =  4  ft.    Required  the  circumference  of  the  axle. 

24.  Required,  in  a  wheel  and  axle,  the  diameter  of   the  wheel,  the 
diameter  of  the  axle  being  10  in.,  the  P  100  Ibs.,  and  the  W 1  ton. 

25.  Why  is  the  rim  of  a  fly-wheel  made  so  heavy? 

26.  Describe  the  hammer,  when  used  in  drawing  a  nail,  as  a  bent  lever^ 
i. «.,  one  in  which  the  bar  is  not  straight. 

27.  Describe  the  four  levers  shown  in  Kg.  46,  when  both  the  load  of 
hay  and  the  weight  are  considered,  respectively,  as  the  W  and  the  P. 


SUM  MARY. 

ALL  machines  can  be  resolved  into  a  few  elementary  forms. 
Of  these  there  are  six,  viz.,  the  lever,  the  wheel  and  axle,  the 
inclined  plane,  the  screw,  the  wedge,  and  the  pulley.  Though 
called  the  mechanical  powers,  they  are  only  instruments  by 
which  we  can  avail  ourselves  of  the  forces  of  nature.  Molar 
energy  or  the  motion  of  masses,  as  of  air,  water,  steam, 
etc.,  is  thus  utilized,  while  the  strength  of  a  horse  does  the 
work  of  many  men.  A  force  of  small  intensity  made  to  act 
through  a  considerable  distance  becomes  one  of  great  intensity 
acting  through  a  small  distance,  and  vice  versa.  By  the  use  of 
the  mechanical  powers,  the  application  of  energy  is  made  more 
convenient,  but  always  some  energy  is  absorbed  in  moving  the 
machine  and  overcoming  friction,  and  hence  prevented  from 
doing  useful  work.  No  machine  can  be  a  source  of  power,  but, 
on  the  contrary,  it  thus  involves  a  loss  of  power.  The  law  of 
machines  is,  that  the  power  multiplied  by  the  distance  through 
which  it  moves  is  equal  to  the  weight  multiplied  by  the  distance 
through  which  it  moves,  plus  the  internal  work  involved  in  the 
motion  of  the  machine.  This  law  is  equivalent  to  a  statement 
that  perpetual  motion  is  impossible ;  for  no  known  terrestrial 
source  of  energy  is  exhaustless. 


HISTOKICAL     SKETCH.  97 

The  lever  is  a  bar  resting  at  some  point  on  a  prop  as  a 
center  of  motion.  The  crowbar,  claw-hammer  for  drawing  nails, 
pincers,  windlass,  and  steelyard  are  examples  of  various  classes 
of  levers.  The  compound  lever  consists  of  several  levers,  so 
connected,  that  the  short  arm  of  one  acts  on  the  long  arm  of 
the  next,  as  in  the  hay  scales.  In  the  bent  lever,  the  power 
and  weight  act  in  lines  that  are  not  necessarily  parallel,  but 
still  tend  to  produce  rotation  of  the  lever  about  its  fulcrum,  if 
the  product  of  the  power  by  the  perpendicular  distance  from  its 
line  of  action  to  the  fulcrum  be  not  equal  to  the  weight  multi- 
plied by  the  distance  from  its  line  of  action  to  the  fulcrum. 
These  two  products  are  called  the  moments  about  the  fulcrum. 
If  the  two  moments  are  equal  and  opposite,  the  result  is  equi- 
librium. 

To  the  lever  may  be  reduced  the  wheel  and  axle,  and  the 
pulley.  To  the  inclined  plane  may  be  reduced  the  wedge  and 
the  screw.  The  awl,  vise,  carpenter's  plane,  corkscrew,  and 
stairs  are  common  modifications  of  the  inclined  plane.  The 
blade  of  a  pocket-knife  is  a  familiar  example  of  the  wedge, 
which  itself  is  only  a  movable  inclined  plane.  In  the  applica- 
tion of  these  last  mechanical  powers,  friction  becomes  a  most 
important  and  useful  element ;  and  it  interferes  so  much  with 
the  operation  of  the  simple  machine  alone,  which  should  be  de- 
void of  friction  in  order  to  make  exact  calculation  possible,  that 
it  is  usually  impossible  to  calculate  the  ratio  between  the  power 
applied  and  the  work  accomplished  through  the  medium  of  a 
wedge. 


HISTORICAL     SKETCH. 

SIMPLE  machines  for  moving  large  bodies  are  as  old  as  his- 
tory. The  Babylonians  knew  the  use  of  the  lever,  the  pulley, 
and  the  roller.  The  Romans  were  acquainted  with  the  lever, 
the  wheel  and  axle,  and  the  pulley  (simple  and  compound).  The 
Egyptians,  it  is  thought,  raised  the  immense  stones  used  in 
building  the  Pyramids,  by  inclined  planes  made  of  earth  which 
was  afterward  removed.  Archimedes,  in  the  third  century  B.C., 


98  ELEMENTS     OF     MACHINES. 

discovered  the  law  of  equilibrium  in  the  lever.*  His  investiga- 
tions, however,  were  too  far  in  advance  of  his  time  to  be  fully 
understood,  and  the  teachings  of  Aristotle  were  long  after  ac- 
cepted by  scientific  men.  The  law  of  mechanics,  or  of  Virtual 
Velocities,  as  it  is  called,  was  discovered  by  Galileo. 

*  It  is  often  said  that  Archimedes,  in  allusion  to  the  tremendous  power 
of  the  lever,  asserted  that,  Give  him  a  fulcrum  and  he  could  move  the 
world.  Had  he  been  allowed  such  a  chance,  "  the  fulcrum  being  nine 
thousand  leagues  from  the  center  of  the  earth,  with.  »  power  of  200  Ibs. 
the  geometer  would  have  required  a  lever  12  quadrillions  of  miles  long  and 
the  power  would  have  needed  to  move  at  the  rate  of  a.  cannon-ball  to  lift 
the  earth  one  inch  in  27  trillions  of  years." 


V. 

PRESSURE  OF  LIQUIDS  AND 
•  GASES. 


"THE  waves  that  moan  along  the  shore, 

The  winds  that  sigh  in  blowing, 
Are  sent  to  teach  a  mystic  lore 

Which  men  are  wise  in  knowing.*1 


ANALYSIS. 


2.  LIQUIDS  INFLUENCED 

BY  GRA.VITY. 


(4.)  Specific 
Gravity. 


1.  RULES    CONCERNING 
A  JET. 


1.  LIQUIDS  INFLUENCED  /  (1.)  Law  of  Transmission. 

BY  EXTERN ALPBES-  -j  (2.)  Water  as  a  Mechanical  Power. 
SURE  ONLY.  (  (3.)  Hydrostatic  Press. 

(1.)  Four  Laws  of  Equilibrium. 
(2.)  Rules  for  Computing  Pressure. 
(3.)  Water  Level. 

(a.)  Definition  and  Illustra- 
tion. 

(b.)  Buoyant  Force  of  Liq- 
uids. 

(c.)  To  Find  Specific  Grav- 
ity of  a  Solid. 

(d.)  To  Find  Specific  Grav- 
ity of  a  Liquid. 
(e.)  To  Find  Weight  of  Given 

Volume, 
(f.)   To  Find  Volume  of  Given 

Weight. 
(g.)  To  Find   Volume  of  a 

Body. 

(h.)  Floating  Bodies. 
—DEFINITION  AND  GENERAL  PRINCIPLES. 

(1.)  The  Velocity  that  of  a  Palling  Body. 
(2.)  To  Pind  the  Velocity. 
(3.)  To  Pind  the  Quantity. 

2.  EFFECT  OF  TUBES. 

3.  PLOW  OF  WATER  IN  RIVERS. 

f  (1.)  Overshot. 

4.  WATER-WHEELS.         4  <2'\  Undershot. 

(3.)  Breast. 
I  (4.)  Turbine. 
—DEFINITION  AND  GENERAL  PRINCIPLES. 

1.  AIR-PUMP. 

2.  CONDENSER. 

/  (1.)  Weight. 

3.  PROPERTIES  OF  AIR.  •<  (2.)  Elasticity. 

(  (3.)  Expansibility. 

(1.)  The  Proof. 

(2.)  Upward  Pressure. 

(3.)  Buoyant  Porce. 

(4.)  Amount  of  Pressure. 

(5.)  Pressure  Varies. 

(6.)  Mariotte's  (or  Boyle's)  Law. 

(7.)  Barometer. 

(1.)  Lifting. 

(2.)  Porcing. 
(  (3.)  Pire-engine. 

6.  SIPHON. 

7.  PNEUMATIC  INSSTAND. 

8.  HYDRAULIC  RAM. 
9. .  ATOMIZER. 

10.  HEIGHT  OF  THE  ATMOSPHERE. 


4.  PRESSURE     OF    THE 
AIR. 


5.  PUMPS. 


PRESSURE  OF  LIQUIDS  AND  GASES. 


1.    HYDROSTATICS. 

HYDROSTATICS  treats  of  liquids  at  rest.  Its  princi- 
ples apply  to  all  liquids;  but  water,  on  account  of 
its  abundance,  is  taken  as  the  type. 

1.   Liquids  Influenced  by  External  Pressure  only. 

—(1.)  PASCAL'S  LAW.*  Liquids  transmit 
pressure  equally  in  all  directions,  and  this 
acts  at  right  angles  upon  the  surface 
pressed. 

As  the  particles  of  a  liquid  move  freely 
among  themselves,  there  is  no  loss  by 
friction,  and  any  force  will  be  transmitted 
equally  upward,  downward,  and  side  wise. 
Thus,  if  a  bottle  be  filled  with  water  and 
a  pressure  of  1  Ib.  be  applied  upon  the 
cork,  it  will  be  communicated  from  par- 
ticle to  particle  throughout  the  water.  If 
the  area  of  the  cork  be  1  sq.  in.,  the  pressure  upon 


Illustration  of 
Pascal's  Law. 


*  This  law  is  named  after  the  celebrated  geometrician,  Blaise  Pascal, 
who  first  enunciated  it  in  1663.  At  first  thought  it  may  seem  impossible 
for  a  pressure  of  1  Ib.  to  produce  a  pressure  of  100  Ibs. ;  but  it  should  be 
remembered  that  the  general  law  of  mechanics  applies  to  liquids  as  well  as 
solids.  If  a  force  of  1  Ib.  on  i  sq.  in.  should  came  motion  by  pressing 
through  the  medium  of  a  liquid  on  100  sq.  in.,  the  velocity  of  the  body 
moved  will  be  only  ii5th  of  that  of  the  body  applying  the  pressure. 


102   L^PKE1SS;UR;E:QF     LIQUIDS     AND     GASES. 


PIG.  65. 


Tube  with  Cylinder  of  Lead. 


FIG.  66. 


Bent  Tube  Full  of  Water. 


any  sq.  in.  of  the  glass  at  n,  a,  b,  or  c,  will  be  1  Ib. 
If  the  inside  surface  of  the  bottle  be  100  sq.  in.,  a 
pressure  of  1  Ib.  upon  the  cork  will  produce  a  force 
of  100  Ibs.,  tending  to  burst  the  bottle. 

Illustrations. — The 
P  transmission  of  press- 
ure by  liquids  under 
soms  circumstances,  is 
more  perfect  than  by  solids.  Let  a  straight  tube, 
AB,  be  filled  with  a  cylinder  of  lead,  and  a  piston 
be  fitted  to  the  end  of 
the  tube.  If  a  force  be 
applied  at  P,  it  will  be 
transmitted  to  0  with- 
out sensible  loss.  If  in- 
stead, we  use  a  bent 
tube,  the  force  will  be  transmitted  in  the  lines  of  the 

arrows,  and  will  act  on 
Pbut  slightly.  If,  how- 
ever, we  fill  the  tube 
with  water,  the  force 
will  be  transmitted 
without  diminution.* 
Take  a  glass  bulb  and 
stem  full  of  water,  as  in 
Fig.  67.f  1^  you  are 
pounding  with  a  Glass  Buib.  careful  to  let  the  stem 

slip  loosely  through  your  fingers  as  the  bulb  strikes, 

*  With  cords,  pulleys,  levers,  etc.,  we  lose  often  more  than  one  half  of 
the  force  by  friction;  but  this  "liquid  rope"  transmits  it  with  no  appre- 
ciable loss. 

t  The  process  of  filling  such  bulbs  is  shown  on  p.  248.    They  are  cheaply 


FIG.  67. 


HYDROSTATICS. 


103 


FIG.  68. 


Rupert's  Drop  in  Vial. 


you  may  pound  it  upon  a  smooth  surface.    The  force 

of  the  blow  is  instantly  transmitted  from  the  thin 

glass  to  the  water,  which  is  almost  incompressible, 

and  this  makes  the  bulb 

nearly  as  solid  as  a  ball  of 

glass.      If  a  Rupert's  drop 

be  held  in  a  closed  vial  of 

water  so  as  not  to  touch 

the    glass    (Fig.    68),    and 

the  tapering  end  be  broken, 

the    water    will    transmit 

the  concussion  and  shatter 

the  vial. 

(2.)  WATER  AS  A  MECHANICAL  POWER. — Take  two 

cylinders,  P  and  p  (Fig.  69),  fitted  with  pistons  and 

filled  with  water.    Let  the  area  of  p  be  1  sq.  in.  and 

that  of  P  be  100  sq.  in. 
Then  a  downward  press- 
ure of  1  Ib.  on  each  sq.  in. 
of  the  small  piston  will 
produce  an  upward  press- 
ure of  1  Ib.  on  each  sq.  in. 
of  the  large  piston.  H&nce 
a  P  of  1  IV.  moving  at  a 

rate  of  1  in.  per  second  will  lift  a  W  of  100  Ibs.  at 

a  rate  of  yfoth  of  an  inch  per  second.* 

purchased  of  apparatus  dealers,  and  explain  not  only  this  point,  but  also  the 
method  of  filling  thermometers. 

*  Pascal  announced  the  discovery  of  this  principle  in  the  following 
words :  "  If  a  vessel  full  of  water  closed  on  all  sides  has  two  openings,  the 
one  a  hundred  times  as  large  as  the  other,  and  if  each  be  supplied  with  a 
piston  which  fits  exactly,  a  man  pushing  the  small  piston  will  exert  a  force 
which  will  balance  that  of  a  hundred  men  pushing  the  large  one." 


FTG.  69. 


Principle  of  the  Hydrostatic  Press. 


104        PRESSURE     OF     LIQUIDS     AND     GASES. 

(3.)  THE  HYDROSTATIC  PRESS  (Fig.  70)  utilizes  the 
principle  just  explained.  As  the  workman  depresses 
the  lever  0,  he  forces  down  the  piston  a  upon  the 
water  in  the  cylinder  A.  The  pressure  is  transmitted 
through  the  bent  tube  of  water  d  under  the  piston 


PIG.  70 


The  Hydrostatic  Press. 

(7,  which  lifts  up  the  platform  K,  and  compresses 
the  bales.  If  the  area  of  a  be  1  in.  and  that  of  C 
100  in.,  a  force  of  100  Ibs.  will  balance  10,000  Ibs. 
The  handle  is  a  lever  of  the  second  class.  If  the  dis- 
tance of  the  hand  from  the  pivot  be  ten  times  that 
of  the  piston,  a  P  of  100  Ibs.  will  produce  a  force 


HYDROSTATICS. 


105 


of  1,000  Ibs.  at  a.  This  will  become  100,000  Ibs.  at 
(7.*  According  to  the  principle  of  mechanics,  PxPd 
=  Wx  Wdj  the  platform  will  ascend  T-oVo  of  the  dis- 
tance the  hand  descends.  This  machine  is  used  for 
baling  hay  and  cotton,  for  launching  vessels,  and  for 
testing  the -strength  of  ropes,  chains,  etc. 

2.  Liquids  Influenced  by  Gravity.— The  lower 
part  of  a  vessel  of  water  must  bear  the  weight  of 
the  upper  part.  Thus  each  particle  of  water  at  rest 
is  pressed  downward  by  the  weight  of  the  minute 
column  it  sustains.  It  must,  in  turn,  press  in  every 
direction  with  the  same  force,  else  it  would  be  driven 
out  of  its  place  and  the  liquid  would  no  longer  be  at 
rest.  Indeed,  when  a  liquid  is  disturbed  it  comes  to 
rest — i.  e.,  there  is  an  equilibrium  established — only 
when  this  equality  of  pressure  is  produced.  The  fol- 
lowing laws  obtain : 

FIG.  71. 


Transmission  of  Pressure  in  all  Directions.  - 


(1.)  FOUR  LAWS  or  EQUILIBRIUM. — I.  At  any  point 
within  a  liquid  at  rest  the  pressure  is  the  same  in 

*  The  presses  employed  for  raising  the  immense  tubes  of  the  Britannia 
Bridge  across  the  Menai  Strait,  were  each  capable  of  lifting  2,672  tons,  and 
of  "throwing  water  in  a  vacuum  to  a  height  of  nearly  six  miles,  or  over 
the  top  of  the  highest  mountain." 


106        PRESSURE     OF     LIQUIDS     AND     GASES. 


PIG.  72. 


all  directions.  If  the  series  of  glass  tubes  shown  in 
Fig.  71  be  placed  in  a  pail  of  water,  the  liquid  will 
be  forced  up  1  by  the  upward  pressure  of  the  water, 
2  by  the  downward  pressure,  3  by  the  lateral  press- 
ure, and  4  by  the  three  combined  in  different  por- 
tions of  the  tube.  The  water 
will  rise  in  them  all  to  the  same 
height  —  i.  e.,  to  the  level  of  the 
water  in  the  pail. 

II.  The  pressure  increases 
with  the  depth.  Into  a  tall  jar 
full  of  water  put  a  bent  tube 
open  at  both  ends,  as  shown 
in  Fig.  72,  a  little  mercury 
having  been  previously  poured 
in  so  as  to  fill  the  bend.  The 
pressure  of  the  water  forces 
down  the  mercury  in  the  short 
arm,  that  in  the  long  arm  being 
exposed  only  to  the  pressure  of 
the  air.  Suppose  the  difference 
of  level  to  be  an  inch  when  the 
tube  is  lowest.  Then  a  column 
of  mercury  an  inch  long  will 
just  balance  the  weight  of  a 
column  of  water  of  the  same 
thickness  and  nearly  equal  in 

^^    ^   ^    height    Qf     the    ^ 

Let  the  tube  now  be  raised  until  the  surface  of  the 
mercury  in  the  short  arm  is  only  a  fourth  of  its 
previous  distance  from  the  surface.  The  difference 


lucre**  of  Pressure  withDepth. 


HYDROSTATICS. 


107 


of  level  in  the  two  arms  is  now  found  to  be  only  a 
fourth  of  an  inch. 

A  cubic  foot  of  water  weighs  about  62.5  Ibs. 
(1,000  oz.) ;  the  same  volume  of  sea-water  weighs 
64.4  Ibs.;  hence  the  pressure  is  proportionally  greater 
in  sea-water.  At  the  greatest  depth  ever  measured 
in  the  sea,  a  little  over  five  miles,  the  pressure  is 
about  six  tons  on  every  square  inch.  An  empty 

PIG.  73. 


glass  bottle  securely  stoppered  may /be  crushed  before 
sinking  a  hundred  yards. 

III.  The  pressure  does  not  depend  on  the  shape 
or  size  of  the  vessel,  but  on  the  area  and  depth.  In 
the  apparatus  shown  in  Fig.  73,  a  disk  is  held  up  by 
a  string  against  the  bottom  of  an  open  tube,  to 
which  may  be  screwed  vessels  of  different  shapes 
and  sizes,  such  as  A,  B,  or  C.  The  string  is  attached 
to  the  beam  of  a  balance.  In  the  scale  pan  is  put 


108        PRESSUKE     OF     LIQUIDS     AND     GASES. 


Fie,  74. 


such  a  weight,  Jif,  as  to  balance  the  pressure  of  the 
liquid  against  the  disk  when  the  vessel  is  full  up  to 
a  certain  point,  n.  The  addition  of  any  more  liquid 
causes  the  disk  to  sink  and  thus  spill  the  liquid, 
whether  the  large  vessel,  A,  or  the  smaller  ones,  B  or 
(7,  be  used.  Against  the  same  area  at  the  bottom 
the  same  pressure  is  obtained  even  if  A  is  three 
times  B  in  volume,  provided  the 
depth  be  kept  the  same.  If  the 
depth  of  water  be  increased,  M 
has  to  be  increased  in  the  same 
ratio.  Thus  a  pound  of  water 
in  B  may  be  made  to  exert  a 
greater  pressure  at  the  bottom 
than  2  Ibs.  of  water  in  A. 

The  Hydrostatic  Bellows  is  an 
application  of  the  principles  just 
discussed,  and  is  like  the  Hydro- 
static Press  on  a  small  scale.  It 
consists  of  two  boards  connected 
by  a  band  of  leather  and  pro- 
vided with  a  supply  tube  for 
water.  Suppose  the  area  of  C 
(Fig.  74)  to  be  500  sq.  in.,  and 
the  area  across  the  small  tube 
A  to  be  a  single  sq.  in.  Let  the 
stiffness  of  the  leather,  along 
with  an  added  weight  on  (7,  be 
equivalent  to  100  Ibs.  Then 
only  one  fifth  of  a  pound  of  water  in  A  is  needed  to 
balance  the  100  Ibs.  If  at  A  we  use  a  larger  tube 


Hydrostatic  Bellows. 


HYDROSTATICS.  109 

across  which  the  area  is  10  sq.  in.,  then  2  Ibs.  of 
water  in  it  are  required  to  maintain  equilibrium. 
The  surface  of  the  water  in  A  will  be  about  6  in. 
above  (7,  whether  the  large  or  small  tube  is  used. 
If  a  cubic  inch  more  be  poured  into  the  small  tube, 
the  same  quantity  will  be  forced  into  the  bellows,  so 
that  C  rises  ^-th  of  an  inch.  If  a  larger  weight  is 
put  on  (7,  a  higher  column  in  A  is  required,  but  still 
C  can  be  raised  by  any  addition  to  A,  however  small. 

A  strong   cask   fitted  with  a  FIG.  75. 

small  pipe  30  or  40  feet  long, 
if  filled  with  water  will  be  burst 
asunder.*  The  pressure  is  as 
great  as  if  the  tube  were  of  the 
same  diameter  as  the  cask.  In 
a  coffee-pot  the  small  quantity 
of  liquid  in  the  spout  balances 
the  large  quantity  in  the  vessel. 
If  it  were  not  so,  it  would  rise 
in  the  spout  and  run  out. 

IV.  Water  seeks  its  level.  In 
the  apparatus  shown  in  Fig.  76 
the  water  rises  to  the  same 

height  in  the  various  tubes  which  communicate  with 
one  another,  because  so  long  as  the  surfaces  are 
not  at  the  same  level  a  particle  below  any  surface 

*  Suppose  the  pipe  to  have  an  area  of  ts  sq.  in.,  and  to  hold  \  Ib.  of 
water.  The  pressure  on  each  •£>  of  an  inch  of  the  interior  of  the  cask 
would  be  J  Ib.,  or  2,880  Ibs.  on  each  sq.  ft.—  a  pressure  no  common  barrel 
could  sustain.  The  principle  that  a  small  quantity  of  water  will  thus  bal- 
ance another  quantity,  however  large,  or  will  lift  any  weight,  however  great, 
is  frequently  termed  the  "Hydrostatic  Paradox."  It  is  only  an  instance  of 
a  general  law,  and  is  no  more  paradoxical  than  the  action  of  the  lever. 


110        PRESSURE     OF     LIQUIDS     AND     GASES. 


must  be  unequally  pressed  from  opposite  sides,  and 

must  move  until  equi- 
librium is  attained.  In 
Fig.  7  7  £  tank  is  situ- 
ated on  a  hill,  whence 
the  water  is  conducted 
underground  through 
a  pipe  to  the  fountain. 
In  theory  the  jet  will 
rise  to  the  level  of  the  reser- 
voir, but 'in  practice  it  falls 
short,  owing    chiefly   to   the 
friction     in     the     pipe,     the 
weight  of  the  falling  drops, 

Water  in  Communicating  .    .  „    ., 

vessels.  and  the  resistance  of  the  air. 

FIG.  77. 


Construction  of  a  Fountain. 


HYDROSTATICS. 


Ill 


Artesian  Wells.*  —  Let  A  B  and  C  D  represent 
curved  strata  of  clay  impervious  to  water,  and  K  K 
a  layer  of  gravel.  The  rain  falling  on  the  hills  filters 
down  to  C  D,  and  collects  in  this  basin.  If  a  well 
be  bored  at  H,  as  soon  as  it  reaches  the  gravel  the 


Artesian  Well. 

water  will  rush  upward,  under  the  tremendous  lateral 
pressure,  to  the  surface  of  the  ground,  often  spout- 
ing high  in  air.f 

Wells. — Of  the  rain  which  falls  on  the  land,  a  part 
runs  directly  to  the  streams  and  part  soaks  into  the 
soil.  The  latter  portion  may  filter  down  to  an  im- 

*  They  are  so  Jiamed  because  they  have  long  been  used  in  the  province 
of  Artois  (Latin,  Artesium),  Prance.  They  were,  however,  early  employed 
by  the  Chinese  for  the  purpose  of  procuring  gas  and  salt  water. 

t  The  famous  well  at  Grenelle,  Paris,  is  at  the  bottom  of  a  basin  which 
extends  miles  from  the  city.  It  is  about  1,800  feet  deep,  and  furnishes  656 
gallons  of  water  per  minute.  The  two  wells  of  Chicago  are  about  700  feet 
deep,  and  discharge  daily  about  432,000  gallons.  Being  situated  on  the 
level  prairie,  the  force  with  which  the  water  comes  to  the  surface  indicates 
that  it  may  be  supplied  perhaps  from  Bock  River,  100  miles  distant.  There 
are  also  valuable  artesian  wells  at  Louisville,  Kentucky,  and  at  Charleston, 
South  Carolina.  When  the  water  comes  from  a  great  depth,  it  is  generally 
warm. 


112         PRESSURE     OF     LIQUIDS     AND     GASES. 

permeable'  layer  of  rock  or  clay,  and  then  run  along 
till  it  oozes  out  at  some  lower  point  as  a  spring ;  or, 
if  it  can  not  escape,  it  will  collect  in  the  ground.  If 
a  well  be  sunk  into  this  subterranean  reservoir,  the 
water  will  rise  in  it  to  the  level  of  the  source.* 

(2.)  RULES  FOR  COMPUTING  PRESSURE. — I.  To  find 
the  pressure  on  the  bottom  of  a  vessel.  Multiply  the 
area  of  the  base  in  square  feet  by  the  vertical  height 
in  feet,  and  that  product  by  the  weight  of  a  cubic 
foot  of  the  liquid. 

FIG   79. 


The  Curvature  of  a  Water  Level. 

II.  To  find  the  pressure  on  the  side  of  a  vessel. 
Multiply  the  area  of  the  side  in  square  feet  by  half 

*  "From  a  forgetfulness  of  this  principle  the  company  which  dug  the 
Thames  and  Medway  Canal,  England,  incurred  heavy  damages.  Having 
planned  the  canal  to  "be  filled  at  high  tide,  the  salt  water  spread  immedi- 
ately into  all  the  wells  of  the  surrounding  region.  Had  the  canal  been  dug 
a  few  feet  lower,  the  evil  would  have  been  avoided."— ABNOTT. 


HYDROSTATICS.  113 

of  the  vertical  height  in  feet,*  and  that  product  by 
the  weight  of  a  cubic  foot  of  the  liquid.  The  press- 
ure on  the  bottom  of  a  cubical  vessel  of  water  is  the 
weight  of  the  water ;  on  each  side,  one  half ;  and  on 
the  four  sides,  twice  the  weight ;  therefore,  on  the 
five  sides  the  pressure  is  three  times  the  weight  of 
the  water. 

(3.)  WATER-LEVEL. — The  surface  of  standing  water 
is  said  to  be  level — i.  e.,  horizontal  to  a  plumb-line. 
This  is  true  for  small  sheets  of  water,  but  for  larger 
bodies  an  allowance  must  be  made  for  the  spherical 
form  of  the  earth  (Fig.  79).  The  curvature  is  8 
inches  for  the  first  mile,  and  increases  as  the  square 
of  the  distance,  f 

The  spirit-level  is  an  instrument  used  by  builders 
for  leveling.  It  consists  of  a  slightly-curved  glass 
tube  nearly  full  ^  ^ 

of  alcohol,  so  that 
it  holds  only  a 
bubble  of  air. 
When  the  level 
is  horizontal,  the  The  8Pirit-level- 

bubble  remains  at  the  center  of  the  upper  side  of 
the  tube. 

(4.)  SPECIFIC  GRAVITY,  or  relative  weight,  is  the 
ratio  of  the  weight  of  a  substance  to  that  of  the 

*  This  clause  of  the  rule  holds  only  when  the  center  of  gravity  of  the 
side  is  at  half  the  vertical  height.  In  general,  the  depth  of  its  center  of 
gravity  below  the  surface  should  be  used  as  the  multiplier. 

t  For  two  miles  it  is  8  inchesx23=32  inches.  If  one's  eye  were  at  the 
level  of  still  water,  he  could  barely  see  the  top  of  an  object  67  feet  high  at 
a  distance  of  10  miles  in  a  perfectly  clear  atmosphere. 


114        PKESSUKE     OF     LIQUIDS     AND     GASES. 

same  volume  cf  another  substance  taken  as  a  stand- 
ard.    Water  is  taken  as  the  standard*  for  solids  and 
PIQ  gi  liquids,   and  air  for    gases.      A 

cubic  inch  of  sulphur  weighs 
twice  as  much  as  a  cubic  inch 
of  water ;  hence  its  specific 
gravity  =2.  A  cubic  inch  of  car- 
bonic-acid gas  weighs  1.52  times 
as  much  as  the  same  volume  of 
air;  hence  its  specific  gravity = 
1.52.f 

Buoyant  Force  of  Liquids. — 
The  cube  a  b  c  d  is  immersed 

in  water.  The  lateral  pressure  at  a  is  equal  to 
that  at  &,  because  both  sides  are  at  the  same 
depth ;  hence  the  body  has  no  tendency  toward 
either  side  of  the  jar.  The  upward  pressure  at  c  is 
greater  than  the  downward  pressure  at  d,  because 
its  depth  is  greater ;  hence  the  cube  has  a  tendency 
to  rise.  This  upward  pressure  is  called  the  buoyant 
force  of  the  water.  It  is  equal  to  the  weight  of  the 

*  "The  water  must  be  at  39.2°  F.,  its  greatest  density.  In  all  exact 
measurements,  especially  of  standards,  it  is  necessary  to  know  the  tempera- 
ture. For  the  scale  that  is  a  foot  long  to-day  may  be  more  or  less  than  a 
foot  long  to-morrow;  the  measure  that  holds  a  pint  to-day  may  hold  more 
or  less  than  a  pint  to-morrow.  Nay,  more,  these  measures  may  not  be  the 
same  in  two  consecutive  moments.  When  a  carpenter  takes  up  his  rule 
and  applies  it  to  some  object,  the  size  of  which  he  wishes  to  determine,  it 
becomes  in  that  instant  longer  than  it  was  before ;  when  a  druggist  grasps 
his  measuring  glass  in  his  hand  to  dispense  some  of  his  preparations,  the 
glass  increases  in  size.  A  person,  enters  a  cool  room,  and  at  once  it  becomes 
more  capacious,  for  its  walls,  ceiling,  and  floor,  because  of  the  heat  he  im- 
parts, immediately  expand."— DRAPER. 

t  The  term  density  is  often  used  in  the  same  sense  as  specific  gravity, 
especially  in  relation  to  gases. 


HYDROSTATICS. 


115 


FIG.  82. 


liquid  displaced.  For  the  downward  pressure  at  d  is 
the  weight  of  a  column  of  water  whose  area  is  that 
of  the  top  of  the  cube,  and  whose  vertical  height 
is  n  d,  and  the  upward  pressure  at  c  is  equal  to 
the  weight  of  a  column  of  the  same  size  whose 
vertical  height  is  n  c.  The  difference  between  the 
two,  or  the  buoyant  force,  is  the  weight  of  a  volume 
of  water  equal  to  the  size  of  the  cube. 

The  same  principle  is  shown  in  the  "  cylinder-and- 
bucket  experiment."  The  cylinder  a  exactly  fits  in 
the  bucket  &.  When  the  glass  vessel  in  which  the 
cylinder  hangs  is  empty,  the  apparatus  is  balanced 
by  weights  placed  in 
the  scale-pan.  Next, 
water  is  poured  into 
the  glass  vessel.  Its 
buoyant  force  raises 
the  cylinder  and  de- 
presses the  opposite 
scale-pan.  Then  water 
is  dropped  into  the 
bucket;  when  it  is 
exactly  full,  the  scales 
will  balance  again. 
This  proves 
that  a  body  in 
water  is  buoyed 
up  by  a  for^e 
equal  to  the 
weight  of  the  wa»  r  it  displaces."  This  is  called  Ar- 
chimedes' law. 


Cylinder-and-bucket  Experiment. 


116        PRESSURE     OF     LIQUIDS     AND     GASES. 


FIG.  83. 


To  find  the  specific  gravity  of  a  heavy  solid.  Weigh 
the  body  in  air,  and  in  water ;  the  difference  is  the 
weight  of  its  volume  of  water ;  divide  its  weight  in 
air  by  its  loss  of  weight  in  water;  the  quotient  is 
the  specific  gravity.  Thus,  sulphur  loses  one  half  its 
weight  when  immersed  in  water;  hence  it  is  twice 
as  heavy  as  water,  and  its  specific  gravity  =2.* 

To  find  the  specific  gravity  of  a  liquid  ~by  the 
specific-gravity  flask.  This  is  a  bottle 
which  holds  exactly  1,000  grains  of 
water.  If  it  will  hold  1,840  grains  of 
sulphuric  acid,  the  specific  gravity  of 
the  acid  is  1.84. 

To  find  the  specific  gravity  of  a 
liquid  ~by  a  hydrometer.  This  instru- 
ment consists  of  a  glass  tube,  closed  at 
one  end  and  having  at  the  other  a  bulb 
containing  mercury.  A  graduated  scale 
is  marked  upon  the  tube.  The  alcohol- 
ometer, used  in  testing  alcohol,  is  so 
balanced  as  to  sink  in  pure  water  to 
the  zero  point.  As  alcohol  is  lighter  than  water,  the 
instrument  will  descend  for  every  addition  of  spirits. 

*  In  careful  measurements  an  allowance  is  made  for  the  weight  of  the 
air  displaced  by  the  body,  so  that  its  weight  in  a  vacuum  becomes  known. 
Strictly,  it  is  the  weight  in  a  vacuum  that  has  to  be  compared  with  the 
loss  of  weight  in  water.  If  the  body  will  not  sink  in  water,  attach  it  to  a 
heavy  body.  1.  Weigh  the  lighter  body  in  air  (A).  2.  Weigh  the  heavy 
body  in  water  (E).  3.  Weigh  both  together  in  water  (C).  Now  C  is  less 
than  B  because  the  light  body  buoys  up  the  heavy  one ;  I,  e.,  its  weight  A 
is  more  than  balanced,  and  is  replaced  by  an  upward  or  lifting  force=l?— 
<7.  Therefore  the  loss  of  the  light  body  in  wa,teT=A+B—C.'.  spec.  grav.= 

A 
A+B-C' 


Hydrometer. 


HYDROSTATICS.  117 

The  degrees  of  the  scale  indicate  the  percentage  of 
alcohol.  Similar  instruments  are  used  for  determin- 
ing the  strength  of  milk,  acids,  etc. 

To  find  the  weight  of  a  given  volume  of  any  sub- 
stance. Multiply  the  weight  of  one  cubic  foot  of  water 
by  the  specific  gravity  of  the  substance,  and  that 
product  by  the  number  of  cubic  feet. — Example :  What 
is  the  weight  of  three  cubic  feet  of  cork?  Solution: 
1,000  oz.  x. 240*^240  oz. ;  240  oz.  x  3  =  720  oz. 

To  find  the  volume  of  a  given  weight  of  any  sub- 
stance. Multiply  the  weight  of  a  cubic  foot  of  water 
by  the  specific  gravity  of  the  substance,  and  divide 
the  given  weight  by  that  product.  The  quotient  is 
the  required  volume  in  cubic  feet. — Example:  What 
is  the  volume  of  20,000  oz.  of  lead?  Solution:  1,000 
oz.x  11. 36  =  11, 360;  20,000-r-ll, 360  =  1. 76+cu.  ft. 

To  find  the  volume  of  a  body.  Weigh  it  in  water. 
The  loss  of  weight  is  the  weight  of  the  displaced 
water.  Then,  as  a  cubic  foot  of  water  weighs  1,000 
oz.,  we  can  easily  find  the  volume  of  water  displaced. 
—  Example :  A  body  loses  1 0  oz.  on  being  weighed 
in  water.  The  displaced  water  weighs  10  oz.  and 
is  riir  of  'a  cubic  foot ;  this  is  the  exact  volume  of 
the  body. 

*  TABLE  or  SPECIFIC  Q-RAVITT.     (See  "Chemistry,"  p.  288.) 


Iridium 21.80 

Platinum 21.53 

Gold 19.34 

Mercury ...  13.59 

Lead 11.36 

Silver  10.50 

Copper 8.90 

Cast-iron 7.21 


Zinc 7.15 

Diamond. ..  .about  3.50 

Flint  Glass 2.76 

Chalk 2.65 

Sulphur 2.00 

Ice   93 

Potassium 86 

Quicklime 80 


Pine  Wood 66 

Cork 24 

Sulphuric  Acid 1.84 

Water  from  Dead  Sea.  1.24 

Milk 1.03 

Sea-water 1.03 

Absolute  Alcohol .79 

N 


118        PRESSURE     OF     LIQUIDS     AND     GASES. 

Floating  Bodies.— A  body  will  float  in  water  when 
its  weight  is  no.t  greater  than  that  of  an  equal  vol- 
ume of  the  liquid,  and  its  weight 
always  equals  that  of  the  fluid 
displaced.  An  egg  dropped  into 
a  glass  jar  half  full  of  water 
(Fig.  84)  sinks  directly  to  the 
bottom.  If,  by  means  of  a  funnel 
with  a  long  tube,  we  pour  brine 
beneath  the  water,  the  egg  will 
rise.  We  may  vary  the  experiment  by 
not  dropping  in  the  egg  until  we  have 
half  filled  the  jar  with  the  brine.  The 
egg  will  then  fall  to  the  center,  and  there 
float.  Almost  any  solid,  if  dissolved  in 
water,  fills  the  pores  of  the  water  with- 
out adding  much  to  its  volume.  This  Eggm  water, 
increases  its  density  and  buoyant  power.  A  person 
can  therefore  swim  more  easily  in  salt  than  in 
fresh  water.* — An  iron  ship  will  not  only  float 
itself,  but  also  carry  a  heavy  cargo,  because  it 
displaces  a  great  volume  of  water. — A  body  floating 
in  water  has  its  center  of  gravity  at  the  lowest 
point,  when  it  is  in  stable  equilibrium,  f — Fishes 
have  air-bladders,  by  which  they  can  rise  or  sink 

*  Bayard  Taylor  says  that  he  could  float  on  the  surface  of  the  Dead 
Sea,  with  a  log  of  wood  for  a  pillow,  as  comfortably  as  if  lying  on  a  spring 
mattress.  Another  traveler  remarks,  that  on  plunging  in  he  was  thrown 
out  again  like  a  cork ;  and  that  on  emerging  and  drying  himself,  the  crys- 
tals of  salt  which  covered  his  body  made  him  resemble  an  "animated  stick 
of  rock-candy." 

t  Herschel  tells  an  amusing  story  of  a  man  who  attempted  to  walk  on 
water  by  means  of  large  cork  boots.  Scarcely,  however,  had  he  ventured 


HYDROSTATICS.  119 

at  pleasure.*  By  compressing  the  air-bladder,  the 
fish  diminishes  the  volume  of  its  own  body.  The 
buoyant  effect  of  the  water  is  correspondingly  de- 
creased and  the  fish  descends.  By  relaxing  the  com- 
pression on  the  bladder,  the  air  in  it  expands  and 
the  fish  rises. 


PRACTICAL     QUESTIONS. 

1.  Why  can  housekeepers  test  the  strength  of  lye  by  trying  whether  or 
not  an  egg  will  float  on  it? 

2.  How  much  water  will  it  take  to  make  a  gallon  of  strong  brine? 

3.  Why  ought  a  fat  man  to  swim  more  easily  than  a  lean  one  ? 

4.  Why  does  the  firing  of  a  cannon  sometimes  bring  to  the  surface  the 
body  of  a  drowned  person?     Ans.    Because  by  the  concussion  it  shakes  the 
body  loose  from  the  mud  or  any  object  with  which  it  is  entangled. 

5.  Why  does  the  body  of  a  drowned  person  generally  come  to  the  sur 
face  of  the  water  after  a  time  ?   Ans.  Because  the  gases  which  are  generated 
by  decomposition  in  the  body  make*  its  specific  gravity  less. 

6.  If  we  let  bubbles  of  air  pass  up  through  a  glass  of  water,  why  will 
they  become  larger  as  they  ascend? 

7.  What  is  the  pressure  on  a  canal  lock-gate  14  feet  high   and  10  feet 
wide,  when  the  lock  is  full  of  water? 

8.  Will  a  pail  of  water  weigh  any  more  with  a  live  fish  in  it  than 
without? 

9.  If   the   water   filtering   down   through   a  rock  should   collect  in  a 
crevice  an  inch  square  and  250  feet  high,  opening  at  the  bottom  into  a 
closed  fissure  having  20  square  feet  of  surface,  what  would  be  the  total 
pressure  tending  to  burst  the  rock? 

10.  Why  can  stones  in  water  be  moved  so  much  more  easily  than  on 
land? 

11.  Why  is  it  so  difficult  to  wade  in  water  when  there  is  any  current? 

out  ere  the  law  of  gravitation  seized  him,  and  all  that  could  be  seen  was  a 
pair  of  heels,  whose  movements  manifested  a  great  state  of  uneasiness  in 
the  human  appendage  below. 

*  It  was  formerly  thought  that  a  fish  in  water  has  no  weight.  It  Is 
said  that  Charles  II .  of  England  once  asked  the  philosophers  of  his  time  to 
explain  this  phenomenon.  They  offered  many  wise  conjectures,  but  no  one 
thought  of  trying  the  experiment.  At  last  a  simple-minded  man  balanced 
a  vessel  of  water,  and  on  adding  a  fish,  found  it  weighed  just  as  much  as 
if  placed  on  a  dry  scale-pan. 


120        PRESSURE     OF     LIQUIDS     AND     GASES. 

12.  "Why  is  a  mill-dam  or  canal  embankment  small  at  the  top  and  large 
at  the  bottom? 

13.  In  digging  canals,  ought  the  engineer  to  take  into  consideration  the 
curvature  of  the  earth? 

14.  Why  does  the  bubble  of  air  in  a  spirit-level  move  as  the  instrument 
is  turned? 

15.  Can  a  swimmer  tread  on  pieces  of  glass  at  the  bottom  of  the  water 
with  less  danger  than  on  land  ? 

16.  "Will  a   vessel   displace   more  water  in  a  fresh  river  than  in  the 
ocean  ? 

17.  "Will  iron  sink  in  mercury? 

18.  The  water  in  the  reservoir  in  New  York  is  about  80  feet  above  the 
fountain  in  the  City  Hall  Park.    "What  is  the  pressure  upon  a  single  inch  of 
the  pipe  at  the  latter  point? 

19.  "Why  does  cream  rise  on  milk? 

20.  There  is  a  story  told  of  a  Chinese  boy  who  accidentally  dropped 
his  ball  into  a  deep  hole  where  he  could  not  reach  it.      He  filled   the  hole 
with  water,  but  the  ball  would  not  quite  float.      He  finally  thought  of  a 
successful  expedient.    Can  you  guess  it? 

21.  "Which  has  the  greater  buoyant  force,  water  or  oil? 

22.  "What  is  the  weight  of  four  cubic  feet  of  cork? 

23.  How  many  ounces  of  iron  will  a  cubic  foot  of  cork  float  in  water? 

24.  What  is  the  specific  gravity  of  a  body  whose  weight  in  air  is  30 
grs.  and  in  water  20  grs. ?    How  much  is  it  heavier  than  water? 

25.  Which  is  heavier,  a  gallon  of  fresh  or  one  of  salt  water? 

26.  The  weights  of  a  piece  of  syenite-rock  in  air  and  water  were  3941.8 
grs.  and  2607.5  grs.    Find  its  specific  gravity. 

27.  A  specimen  of  green  sapphire  from  Siam  weighed  in  air  21.45  grs. 
and  in  water  16.33  grs. ;  required  its  specific  gravity. 

28.  A  specimen  of  granite  weighs  in  air  534.8  grs.,  and  in  water  334.6 
grs. ;  what  is  its  specific  gravity  ? 

29.  What  is  the  volume  of  a  ton  of  iron  ?     A  ton  of  gold  ?     A  ton  of 
copper? 

30.  What  is  the  weight  of  a  cube  of  gold  4  feet  on  each  side  ? 

31.  A  cistern  is  12  feet  long,  6  feet  wide,  and  10  feet  deep;  when  full 
of  water,  what  is  the  pressure  on  each  side  ? 

32.  Why  does  a  dead  fish  always  float  on  its  back? 

33.  A  given  volume  of  water  weighs  62.5  grs.,  and  the  same  volume  of 
muriatic  acid  75  grs.    What  is  the  specific  gravity  of  the  acid? 

34.  A  vessel  holds   10   Ibs.    of  water;   how  much   mercury  would   it 
contain? 

35.  A  stone  weighs  70  Ibs.  in  air  and  50  in  water ;   what  is  its  volume  ? 

36.  A  hollow  ball  of  iron  weighs  10  Ibs. ;   what  must  be  its  volume  to 
float  in  water? 

37.  Suppose  that  Hiero's  crown  was  an  alloy  of  silver  and  gold,  and 
weighed  22  oz.  in  air  and  20*  oz.  in  water.    What  was  the  proportion  of 
each  metal? 


HYDRODYNAMICS.  121 

38.  Why  will  oil,  which  floats  on  water,  sink  in  alcohol? 

39.  A  specific  gravity  bottle  holds  100  gms.  of  water  and  180  gms.  of 
sulphuric  acid.    Required  the  density  of  the  acid. 

40.  What  is  the  density  of  a  body  which  weighs  58  gms.  in  air  and  46 
gms.  in  water? 

41.  What  is  the  density  of  a  body  which  weighs  63  gms.  in  air  and  35 
gms.  in  a  liquid  of  a  density  of  .85? 


II.    HYDRODYNAMICS. 

HYDRODYNAMICS  treats  of  liquids  in  motion.  In 
this,  as  in  Hydrostatics,  water  is  taken  as  the  type. 
In  theory,  its  principles  are  those  of  falling  bodies, 
but  in  practice  they  can  not  be  relied  upon  except 
when  verified  by  experiment.  The  discrepancy 
arises  from  changes  of  temperature  which  vary  the 
fluidity  of  the  liquid,  from  friction,  the  shape  of  the 
orifice,  etc. 

1.    Rules  Concerning  a  Jet. — (1.)     THE  VELOCITY 

OF  A  JET  IS  THE  SAME  AS  THAT  OF  A  BODY  FALLING 
FROM  THE  SUEFACE  OF  THE  WATER.  We  Can  S66  that 

this  must  be  so,  if  we  recall  two  principles :  First, 
"  a  jet  will  rise  to  the  level  of  its  source ; "  and 
second,  "to  elevate  a  body  to  any  height,  it  must 
have  the  same  velocity  that  it  would  acquire  in  fall- 
ing that  distance."  It  follows  that  the  velocity  of  a 
jet  depends  on  the  height  of  the  liquid  above  the 
orifice. 

(2.)      TO    FIND    THE    VELOCITY    OF    A     JET    OF    WATER, 

use  the  4th  equation  of  falling  bodies,  v*  =  2gh,  in 
which  h  is  the  distance  of  the  orifice  below  the  sur- 
face of  the  water. — Example:  The  depth  of  water 


122        PRESSURE     OF     LIQUIDS     AND     GASES. 

above  the  orifice  is  49   feet;    required  the  velocity 


Substituting,  v=^/2  x  32  x-49  =  56  feet. 

(3.)     TO    FIND    THE    QUANTITY    OF  WATER    DISCHARGED 

IN  A  GIVEN  TIME,  multiply  the  area  of  the  orifice  by 
the  velocity  of  the  water,  and  that  product  by  the 
number  of  seconds.  —  Example  :  What  quantity  of 
water  will  be  discharged  in  5  seconds  from  an  orifice 
having  an  area  of  £  sq.  foot  at  an  average  depth  of 
49  feet  ?  At  that  depth,  v  =  ^2  x  32  x  49  =  56  feet 
per  second  ;  multiplying  by  |,  we  have  2  8  cubic  feet 
discharged  in  one  second  and  140  -cubic  feet  in  five 
seconds.*  In  practice,  much  less  than  this  can  be 
realized. 

2.  Effect  of  Tubes.  —  If  we  examine  a  jet  of 
water,  we  see  its  size  is  decreased  just  outside  the 
orifice  to  about  two  thirds  that  at  the  opening.  This 
neck  is  called  the  vena  contracta,  and  is  caused  by 
the  water  producing  cross  currents  as  it  flows  from 
different  directions  toward  the  orifice.  If  a  tube  of 
a  length  twice  or  thrice  the  diameter  of  the  opening 
be  inserted,  the  water  will  adhere  to  the  sides  so 
that  there  will  be  no  contraction,  and  the  flow  be 
increased  to  about  80  per  cent,  of  the  theoretical 
amount.  If  the  tube  be  conical,  and  inserted  with 
the  large  end  inward,  the  discharge  may  be  aug- 
mented to  95  per  cent.  ;  and  if  the  outer  end  be 


*  If ,  at  a  foot  below  the  surface,  an  opening  will  furnish  1  gallon  per 
minute,  to  double  that  quantity  the  opening  must  be  4  feet  below  the  top. 
Again,  if  a  certain  power  will  force  through  a  nozzle  of  a  fire-engine  a 
given  quantity  of  water  in  a  minute,  to  double  the  quantity  the  power 
must  be  quadrupled. 


HYDRODYNAMICS.  123 

flaring,  it  may  reach  98  per  cent.  Long  tubes 
or  short  angles,  by  friction,  diminish  the  flow  of 
water. 

3.  Flow   of  Water   in   Rivers,— A   fall   of  three 
inches  per  mile  is  sufficient  to  give  motion  to  water, 
and  produce  a  velocity  of  as  many  miles  per  hour. 
The  Ganges  descends  but  800  feet  in  1,800  miles. 
Its  waters  require  a  month  to  move  down  this  long 
inclined  plane.*    A  fall  of  three  feet   per  mile  will 
make  a  mountain  torrent.     The  current  moves  more 
swiftly  at  the  center  than  near  the  shores  or  bottom 
of  a  channel,  since  there  is  less  friction. 

4.  Water-wheels    are    machines    for   using   the 
force  of  falling  water.     By  bands  FIG  85 

or  cog-wheels  the  motion  of  the 
wheel  is  conducted  from  the  axle 
into  the  milLf 

The  OVERSHOT  WHEEL  has  on  its 
circumference  a  series  of  buckets 
which    receive    the    water  flowing 
from  a  sluice,  C.     These  hold  the 
water  as  they  descend  on  one  side, 
and  empty  it  as  they  come  up  ori  the  other.     Over- 
shot wheels  are  valuable  where  a  great  fall  can  be 
secured,  since  they  require  but  little  water.    If  W  de- 
notes the  weight  of  the  water  and  h  the  distance  it 


*  "The  fall  of  800  feet  would  theoretically  give  a  velocity  of  more 
than  150  miles  per  hour.  This  is  reduced  by  friction  to  about  three  miles." 

t  The  principle  is  that  of  a  lever  with  the  P  acting  on  the  short  arm. 
In  this  way  the  movement  of  the  slow  creaking  axle  reappears  in  the 
swiftly  buzzing  saw  or  flying  spindle. 


124        PKESSURE     OF     LIQUIDS     AND     OASES. 


falls,  then  the  total  work  =  Wh.     Of  this  amount,  75 
per  cent,  can  be  made  available  under  good  conditions. 


FIG.  86. 


FIG.  87. 


Undershot  Wheel. 


Breast  wheel. 


FIG.  88. 


.The  UNDERSHOT  WHEEL  has  projecting  boards,  or 
floats,  which  receive  the  force  of  the  current.     It  is 

of  use  where  there 
is  little  fall  and  a 
large  quantity  of 
water.  It  utilizes 
not  more  than  25 
per  cent,  of  the 
energy  of  the  wa- 
ter. 

The  BREAST- 
WHEEL  (Fig.  87)  is 
a  medium  between 
the  two  kinds  al- 
ready named. 

The   TURBINE 
WHEEL   is    placed 
horizontally  and 
immersed    in  the 
water,     In  Fig.  88,  G  is  the  dam  and  DA  the  spout 


Turbine  Wheel. 


HYDRODYNAMI  OS. 


125 


FIG.  89. 


by  which  the  water  is  furnished.  E  is  a  scroll-like 
casing  encircling  the  wheel,  and  open  at  the  center 
above  and  below.  The  axis  of  the  wheel  is  the  ver- 
tical cylinder  B,  from  which  radi- 
ate plane-floats  against  which  the 
water  strikes.  This  form  utilizes 
as  high  as  90  per  cent,  of  the 
energy.  F  is  a  band-wheel  which 
conducts  the  power  to  the  ma- 
chinery. 

The    principle    of    the    unbal- 
anced pressure   of  a  column  of 
water  may  also  be  employed.    It 
is  illustrated  in  Barker's  Mill  or 
Reaction-wheel.*      This    consists 
of  an  upright  cylinder  with  hori- 
zontal arms,  on  the  opposite  sides 
of  which  are  small  apertures.     It 
rests  in  a  socket,  so  as  to 
revolve  freely.    Water  is 
supplied     from     a    tank 
above.   If  the  openings  in 
the  arms  are  closed,  when 
the  cylinder  is  filled  with 
water    the     pressure    is 

equal  in  all  directions  and   the  machine   is   at  rest. 
If  now  we  open  an  aperture,  the  pressure  is  relieved 

*  Bevolving  fire-works  and  the  whirligig,  used  for  watering  lawns  and 
as  an  ornament  in  fountains,  are  constructed  on  the  same  principle.— An 
ingenious  pupil  can  easily  construct  a  Reaction-wheel  of  straws  or  quills, 
pouring  the  water  into  the  upright  tube  by  means  of  a  pitcher,  or  admitting 
it  slowly  through  a  siphon  from  a  pail  of  water  placed  on  a  table  above. 


Barker's  Mill. 


126        PKESSUKE     OF     LIQUIDS     AND     GASES. 

on  that  side,  and  the  arm  flies  back  on  account  of 
the  unbalanced  pressure  of  the  column  of .  water 
above. 

5.  Waves  are  produced  by  the  friction  of  the 
wind  against  the  surface  of  the  water.  The  wind 
raises  the  particles  of  water  and  gravity  draws  them 
back  again.  They  thus  vibrate  up  and  down,  but  in 
deep  water  the  liquid  mass  does  not  advance.  The 
forward  movement  of  the  wave  is  an  illusion.  The 
form  of  the  wave  progresses  like  the  apparent  motion 
of  the  thread  of  the  screw  which  we  turn  in  our 
hand,  or  the  undulations  of  a  rope  or  carpet  which 
is  shaken,  or  the  stalks  of  grain  which  bend  in  bil- 
lows as  the  wind  sweeps  over  them. 

The  corresponding  parts  of  different  waves  are 
said  to  be  like  phases.  Thus,  in  Fig.  90,  A  and  E, 

FIG.  90. 


B  and  Fr  C  and  G  are  like  phases.  The  distance 
between  two  like  phases,  or  between  the  crests  of 
two  succeeding  waves,  is  called  a  wave-length.  Thus 
the  distance  AE,  or  BF,  or  CO-  is  a  wave-length. 
Opposite  phases  are  those  parts  which  are  vibrating 
in  opposite  directions,  as  E  and  (7,  or  B  and  D.  The 
successive  particles  of  water  move  each  in  an  ellipse, 
and  in  regular  succession,  so  that  when  a  particle  at 


HYDRODYNAMICS. 


127 


E  is  moving  forward,  one  at  C  is  moving  backward, 
one  at  B  upward,  and  one  at  D  downward.  This  is 
easily  observed  at  sea.* 

COMPOSITION  OF  WAVE  MOTION. — A  tide-wave  may 
be   setting  steadily   toward    the   west ;    waves   from 


FIG.  91. 


Interference  of  Waves. 

distant  storms  may  be  moving  upon  this  ;  and,  above 
the  whole,  ripples  from  the  breeze  then  blowing  may 
diversify  the  surface.  These  different  systems  of 

*  Near  the  shore  the  osciltetions  become  shorter;  the  lower  particles 
being  checked  in  their  elliptic  motion  by  the  friction  on  the  sandy  beach, 
the  front  becomes  well-nigh  vertical,  and  the  tipper  part  curls  over  and 
falls  beyond.  The  size  of  "  mountain  billows  "  has  been  exaggerated.  Along 
the  coast  in  a  gorge  they  may  reach  90  feet,  but  in  the  open  sea  the  highest 
wave,  from  the  deepest  "trough"  to  the  very  topmost  "crest,"  rarely 
ures  over  30  feet. 


128         PKESSURE    OF    LIQUIDS    AND    GASES. 

waves  will  compound  into  a  resultant  system  in  ac- 
cordance with  the  following  general  principles :  If  any 
two  systems  coincide  with  like  phases, — the  crest  of  one 
meeting  the  crest  of  the  other,  and  the  furrow  of  one 
meeting  the  furrow  of  ^the  other, — the  resulting  wave 
will  have  a  height  equal  to  the  sum  of  the  two.  If  any 
two  systems  coincide  with  opposite  phases, — the  hollow 
of  one  striking  the  crest  of  another, — the  height  will  be 
the  difference  of  the  two.  Thus,  if  in  two  systems  hav- 
ing the  same  wave-length  and  height,  one  is  exactly 
half  a  length  behind  the  other,  they  will  destroy  each 
other.  This  is  termed  the  interference  of  waves.* 

The  manner  in  which  different  waves  move 
among  and  upon  one  another,  is  seen  by  dropping  a 
handful  of  stones  in  water  and  watching  the  waves 
as  they  circle  out  from  the  various  centers  in  ever- 
widening  curves.  In  Fig.  91  is  shown  the  beautiful 
appearance  these  waves  present  when  reflected  from 
the  sides  of  a  vessel. 

*  "  In  the  port  of  Batsha  the  tidal-wave  comes  up  by  two  distinct  chan- 
nels so  unequal  in  length  that  their  time  of  arrival  varies  by  six  hours. 
Consequently  when  the  crest  of  high  water  reaches  the  harbor  by  one 
channel,  it  meets  the  low  water  returning  by  the  other,  and  when  these 
opposite  phases  are  equal,  they  neutralize  each  other,  so  that  at  particular 
seasons  there  is  no  tide  in  the  port,  and  at  other  times  there  is  but  one 
tide  per  day,  and  that  equal  to  the  difference  between  the  ordinary  morn- 
ing and  evening  tide."— Lloyd's  Wave  Theory. 

Another  striking  example  of  interference  of  tide- waves  is  seen  in  the 
immediate  neighborhood  of  New  York.  The  tide-wave  from  the  ocean, 
coming  from  the  south-east,  divides,  a  part^passing  up  New  York  Bay,  and 
another  part  sweeping  around  and  turning  westward  through  Long  Island 
Sound.  The  meeting-place  of  these  two  branches  is  at  Hell  Gate,  the  narrow- 
est ship-channel  between  Long  Island  and  New  York.  If  a  wall  were  built 
across  Hell  Gate,  the  water  on  one  side  would  sometimes  be  five  feet  above 
that  on  the  other.  In  the  absence  of  such  a  wall,  the  current  surges  with 
great  rapidity  under  the  Brooklyn  Bridge,  alternately  in  opposite  directions. 


PNEUMATICS.  129 


PRACTICAL     QUESTIONS. 

1.  Two  faucets,  one  8  feet  and  the  other  4  feet  below  the  surface  of  the 
water  in  a  cistern,  are  kept  Open  for  a  minute.    How  many  times  as  much 
water  can  be  drawn  from  the  first  as  the  second? 

2.  How  much  water  will  be  discharged  per  second  from  a  short  pipe 
having  a  diameter  of  4  inches  and  a  depth  of  48  feet  below  the  surface  of 
the  water? 

3.  When  we  pour  molasses  from  a  jug,  why  is  the  stream  so  much 
larger  near  the  nozzle  than  at  some  distance  from  it? 

4.  Ought  a  faucet  to  extend  into  a  barrel  beyond  the  staves? 

5.  What  would  be  the  effect  if  both  openings  in  one  of  the  arms  of 
Barker's  Mill  were  on  the  same  side? 


III.    PNEUMATICS. 

PNEUMATICS  treats  of  the  general  properties  and 
the  pressure  of  gases.  Since  the  molecules  move 
among  themselves  more  freely  even  than  those  of 
liquids,  the  conclusions  which  we  have  reached  with 
regard  to  transmission  of  pressure,  buoyancy,  and 
specific  gravity  apply  also  to  gases.  Since  air  is  the 
most  abundant  gas,  it  is  taken 

FIG.  92. 

as  the  type  of  the  class,  just 
as  water  is  of  liquids. 

1.  The  Air-pump  is  shown 
in    its    essential    features    in 
Fig.  92.    A  is  a  glass  receiver 
standing  on  an  oiled  pump-  B\ 
plate.    The  tube  D  connecting  The 

the  receiver  with  the  cylinder, 

is  closed  by  the  valve  E,  opening  upward.  There  is 
a  second  valve,  P,  in  the  piston,  also  opening  up- 
ward. Suppose  the  piston  is  at  the  bottom  and  both 


130        PRESSURE     OF     LIQUIDS     AND     GASES. 

valves  shut.  Let  it  now  be  raised,  and  a  vacuum 
will  be  produced  in  the  cylinder ;  the  expansive  force 
of  the  atmosphere  in  the  receiver  will  open  the  valve 
E  and  drive  the  air  through  to  fill  this  empty  space. 
When  the  piston  descends,  the  valve  E  will  close, 
while  the  valve  P  will  open,  and  the  air  will  pass 
up  above  the  piston.  On  elevating  the  piston  a  sec- 
ond time,  this  air  is  removed  from  the  cylinder, 
while  the  air  from  the  receiver  passes  through  as 
before.  At  each  stroke  a  portion  of  the  atmosphere 
is  drawn  off;  but  the  expansive  force  becomes  less 
and  less,  until  finally  it  is  insufficient  to  lift  the 
valves.  For  this  reason  a  perfect  vacuum  can  not  be 
obtained. 

2.  The   Condenser,   in  construction,  is  the  same 
as  the  air-pump,  except  that  the  valve  opens  inward 
instead  of  outward.    Instead  of  exhausting,  it  forces 
more  air  into  a  vessel.* 

3.  Properties  of  Air. — (1.)  WEIGHT. — Exhaust  the 
air  from  a  flask  which  holds   100  cubic  inches,  and 
then  balance  it.      On  turning  the  stop-cock,  the  air 
will   rush   in   with   a  whizzing   noise   and   the   flask 


*  The  practical  applications  of  this  pump  are  numerous.  The  soda 
manufacturer  uses  it  to  condense  carbonic  acid  in  soda-water  reservoirs.— 
The  engineer  employs  it  in  laying  the  foundations  of  bridges.  Large  tubes 
or  caissons  are  lowered  to  the  bed  of  the  stream,  and  air  being  forced  in, 
drives  out  the  water.  The  workm'en  are  let  into  the  caissons  by  a  sort  of 
trap,  and  work  in  this  condensed  atmosphere.— Pneumatic  dispatch-tubes 
contain  a  kind  of  train  holding  the  mail,  and  back  of  this  a  piston  fitting 
the  tube  Air  is  forced  in  behind  the  piston  or  exhausted  before  it,  and  so 
the  train  is  driven  through  the  tube  at  a  high  speed.— In  the  Westinghouse 
air-brake,  condensed  air  is  forced  along  a  tube  running  underneath  the 
cars,  and  by  its  elastic  force  drives  the  brakes  against  the  wheel. 


PNEUMATICS. 


131 


FIG.  93. 


Weighing  Air. 


Pio.  94. 


will  descend  (Fig.  93).      It  will  require  31  grains  or 
more  to  restore  the  equipoise. 

(2.)  ELASTICITY  is  shown  in  a  pop- 
gun. We  compress  the  atmosphere  in 
the  barrel  until  the  elastic  force  drives 
out  the  stopper  with  a  loud  report.  As 
we  crowd  down  the  piston  we  feel  the 
elasticity  of  the  air  yielding  to  our 
strength,  like  a  bent  spring. — The  bottle- 
imps,  or  Cartesian  divers,  illustrate  the 
same  property.  Fig.  94  represents  a 
simple  form  of  this  apparatus.  The 
cover  of  a  fruit-jar  is  fitted  with  a 
tube,  which  is  inserted  in  a  syringe-bulb.  The  jar 
is  filled  with  water  and  the  diver 
placed  within.  This  is  a  hollow  image 
of  glass,  having  a  small  opening  at  the 
end  of  the  curved  tail.  If  we  squeeze 
the  bulb,  the  air  will  be  forced  into  the 
jar  and  the  water  will  transmit  the 
pressure  to  the  air  in  the  image.  This 
being  compressed,  more  water  will 
enter,  and  the  diver,  thus  becoming 
heavier,  will  descend.  On  relaxing  the 
grasp  of  the  hand  on  the  bulb,  the  air 
will  return  into  it,  the  air  in  the  image 
will  expand,  by  its  elastic  force  driving 
out  the  water,  and  the  diver,  thus 
lightened  of  his  ballast,  will  ascend. 
The  nearer  the  image  is  to  the  bottom,  the  less 
force  will  be  required  to  move  it.  With  a  little  care 


Cartesian  Diver. 


132        PRESSURE     OF     LIQUIDS     AND     GASES. 


FIG.  95. 


it  can  be  made  to  respond  to  the  slightest  pressure, 
and*  will  rise  and  fail  as  if  instinct  with  life.* 

(3.)  EXPANSIBILITY.— Let  a  well- 
dried  bladder  be  partly  filled  with  air 
and  tightly  closed.  Place  it  under 
the  receiver  and  exhaust  the  air. 
The  air  in  the  bladde'r  expanding 
will  burst  it  into  shreds. 

Take  two  bottles  partly  filled  with 
colored  water.  Let  a  bent  tube  be 

Expansibility  of  Air.       mgerted    tightly  ^   A   and    loogely  ^ 

B.  Place  this  apparatus  under  the  receiver  and 
exhaust  the  air.  The  expansive 
force  of  the  air  in  A  will  drive  the 
water  over  into  B.  On  readmitting 
the  air  into  the  receiver,  the  press- 
ure will  return  the  water  into  A. 
It  may  thus  be  driven  from  bottle 
to  bottle  at  pleasure,  f 

mere? 8  fountain  acts  on  the  same  principle,  as  may 


FIG. 


Transfer  of  Liquid  under 
Receiver. 


*  This  experiment  shows  also  the  buoyant  force  of  liquids,  their  trans- 
mission of  pressure  in  every  direction,  and  the  increase  of  the  pressure  in 
proportion  to  the  depth.  The  elasticity  of  the  air,  as  well  as  the  principles 
explained  by  the  Cartesian  diver,  Fig.  94,  may  be  illustrated  in  the  follow- 
ing simple  manner:  Fill  with  water  a  wide-mouth  8-oz.  bottle,  and  also  a 
tiny  vial,  such  as  is  used  by  homeopathists.  Invert  the  vial  and  a  few 
drops  of  water  will  run  out.  Now  put  it  inverted  into  the  bottle,  and  if  it 
does  not  sink  just  below  the  surface  and  there  float,  take  it  out  and  add  or 
remove  a  little  water,  as  may  be  needed.  When  this  result  is  reached,  cork 
the  bottle  so  that  the  cork  touches  the  water.  Any  pressure  on  the  cork 
will  then  be  transmitted  to  the  air  in  the  vial,  as  in  the  image  in 
Fig  94. 

t  Prick  a  hole  in  the  small  end  of  an  egg,  and  place  the  egg  with  the 
big  end  up  in  a  wine-glass.  On  exhausting  the  receiver,  the  bubble  of  air 
in  the  upper  part  of  the  egg  will  drive  the  contents  down  into  the  glass, 
and  on  admitting  the  air  they  will  be  forced  back  again. 


PNEUMATICS. 


133 


Fie.  97. 


FIG.  98. 


be  seen  by  an  examination  of  Fig.  9  7.  Having  removed 
the  jet-tube,  the  upper  globe  is  partly  filled  with 
water.  The  tube  being  then  replaced, 
water  is  poured  into  the  basin  on  top. 
The  liquid  runs  down  the  pipe  at  the 
right,  into  the  lower  globe.  The  air  in 
that  globe  is  driven  up  the  tube  at  the 
left  into  the  upper 
globe,  and  by  its  elas- 
ticity forces  the  water 
there  out  through  the 
jet-tube,  forming  a  tiny 
fountain. 

4.    Pressure   of  the 
Air. — (1.)   THE  PROOF. — 
If  we  cover  a  hand-glass 
with    one   hand,    as   in 
Fig.   98,   on  exhausting   the    air  we    shall    find   the 
pressure    painful.*      Tie 
over    one    end    of    the 
glass    a    piece    of    wet 
bladder.    When  dry,  ex- 
haust  the    air,   and  the 
membrane     will     burst 

With   a   Sharp    report,  f  Magdeburg  Hemispheres. 

*  The  exhaustion  of  the  air  does  not  prodtice  the  pressure  on  the  hand ; 
it  simply  reveals  it.  The  average  pressure  on  each  person  is  16  tons.  It  is 
equal,  however,  on  all  parts  of  the  body,  and  is  counteracted  by  the  air 
within.  Hence  we  never  notice  it.  Persons  who  go  up  high  mountains  or 
go  down  in  diving-bells  feel  the  change  in  the  pressure. 

t  To  show  the  crushing  force  of  the  atmosphere,  take  a  tin  cylinder  15 
inches  long  and  4  inches  in  diameter.  Fit  one  end  with  a  stop-cock  for  the 
exit  of  the  steam.  Put  in  a  little  water  and  boil.  When  the  air  is  entirely 


Hand-glass. 


Hiero's  Fountain. 


FIG.  99. 


134        PRESSURE     OF     LIQUIDS     AND     GASES. 


Fio.  100. 


The  Magdeburg  Hemispheres  are  named  from  the 
city  in  which  Ghiericke,  their  inventor,  resided.  They 
consist  of  two  small  brass  hemispheres,  which  fit 
closely  together,  but  may  be  separated  at  pleasure. 
If,  however,  the  air  be  exhausted  from  within,  several 

persons  will  be  required  to  pull  them 

apart.*      In    whatever  "position    the 

hemispheres  are  held,  the    pressure 

is  the  same. 
(2.)  UPWARD 

PRESSURE. — Fill 

a  tumbler  with 

water,  and  then 

lay  a  sheet  of 

paper  over  the 

top.  Quickly  invert  the  glass, 
and  the  water  will  be  supported 
by  the  upward  pressure  of  the 
air. — Within  the  glass  cylinder, 
Fig.  101,  is  a  piston  working 
air-tight.  Connect  the  nozzle 
above  with  the  air-pump  by 
means  of  a  rubber  tube  and  exhaust  the  air. 
weight  will  leap  up  as  if  caught  by  a  spring. 


Water  held  up  by  Press- 
ure of  Air. 


The  Weight-lifter. 


The 


driven  out,  turn  the  stop-cock.    Pour  cold  water  over  the  outside  to  condense 
the  steam,  when  the  cylinder  will  collapse  as  if  struck  by  a  heavy  blow. 

*  In  the  museum  at  Berlin  the  hemispheres  used  by  Q-uericke  in  his 
experiments  are  preserved.  They  are  of  copper,  and  22  inches  interior 
diameter,  with  a  flange  an  inch  wide,  making  the  entire  diameter  2  feet. 
Accompanying  is  a  Latin  book  by  the  burgomaster  describing  numerous 
pneumatic  experiments  which  he  had  performed,  and  containing  a  wood- 
cut representing  three  spans  of  horses  on  each  side  trying  to  separate  the 
hemispheres. 


PNEUMATICS. 


135 


(3.)  BUOYANT  FOKCE  OF  THE  AIR. — The  law  of 
Archimedes  (p.  114)  holds  true  in  gases.  A  hollow 
sphere  of  glass  or  copper,  Fig.  102,  is  balanced  in 
the  air  by  a  solid  lead  weight,  but  on  being  placed 


FIG.  102. 


FIG.  103. 


Buoyancy  of  Air. 

under  the  receiver  it  steadily 
falls  while  the  air  is  becom- 
ing exhausted.  This  shows 
that  its  weight  was  partly 
sustained  by  the  buoyant 
force  of  the  air. 

(4.)  AT  SEA-LEVEL  THE  PRESS- 
URE OF  THE  AIR  SUSTAINS  A  COL- 
UMN OF  MERCURY  30  INCHES 
HIGH,  OR  OF  WATER  NEARLY  34  FEET  HIGH,  AND  IS  NEARLY 

15  LBS.  PER  SQUARE  INCH.  Take  a  strong  glass  tube 
about  three  feet  in  length,  and  tie  over  one  end  a 
piece  of  wet  bladder.  When  dry,  fill  the  tube  with 
mercury,  and  invert  it  in  a  cup  of  the  same  liquid. 
The  mercury  will  sink  to  a  height  of  about  30 


Torricelli's  Experiment. 


136        PRESSURE     OF     LIQUIDS     AND     GASES: 

inches.  If  the  area  across  the  tube  be  one  square 
inch,  the  metal  will  weigh  about  14.7  Ibs.  The 
weight  of  the  column  of  mercury  is  equal  to  the 
downward  pressure  on  each  square  inch  of  the 
surface  of  the  mercury  in  the  cup.  Hence  we 
conclude  that  the  pressure  of  the  atmosphere  is 
14.7  Ibs.  per  square  inch,  and  will  balance  a  column 
of  mercury  30  inches  high.  As  water  is  13£  times 
lighter  than  mercury,  the  same  pressure  would 
balance  a  column  of  that  liquid  13£  times  higher, 
or  33}  feet.* 

(5.)  PRESSURE  OF  THE  AIR  VARIES.!  Changes  of 
temperature,  moisture,  etc.,  continually  vary  the 
density  of  the  air,  and  change  the  height  of  the 
column  of  liquid  it  can  support.  The  pressure  also 
increases  with  the  depth.  Hence,  in  a  valley  the 
column  of  mercury  stands  higher  than  on  a  mountain. 
The  pressure  of  the  atmosphere  is  29.92  inches  only 
at  the  level  of  the  sea,  and  at  the  temperature  of 
melting  ice  at  latitude  45°.  The  variation  due  to 
latitude  is  very  slight;  that  due  to  temperature  is 
greater,  and  that  due  to  elevation  is  greatest.  Ob- 
servations on  the  barometer  at  any  given  station 

*  On  account  of  the  unwieldly  length  of  the  tube  required  to  exhibit 
the  column  of  water,  it  is  not  easy  to  verify  this.  It  may,  however,  be 
prettily  illustrated.  Pour  on  the  mercury  in  the  cup  (Fig.  103)  a  little 
water  colored  with  red  ink.  Then  raise  the  end  of  the  tube  above  the  sur- 
face of  the  metal,  but  not  above  that  of  the  water;  this  will  rise  in  the 
tube,  the  mercury  passing  down  in  beautifully-beaded  globules.  The  mer- 
curial column  is  only  30  inches  high,  while  the  water  will  fill  the  tube. 
Finish  the  experiment  by  puncturing  the  bladder  with  a  pin,  when  the 
water  will  instantly  fall  to  the  cup  below. 

t  We  live  at  the  bottom  of  an  aerial  ocean  whose  depth  is  greater  than 
that  of  the  deepest  sea.  Its  invisible  currents  surge  round  us  on  every  side. 


PNEUMATICS. 


137 


are    generally    "reduced"    to    what    they    would    be 
under  the  standard  conditions  just  mentioned. 

(6.)  MARIOTTE'S  LAW.* 
— Fig.  104  represents  a 
long,  bent  glass  tube  with 
the  end  of  the  short  arm 
closed.  Pour  mercury  into 
the  long  arm  until  it  rises 
to  the  point  marked  zero.f 
It  stands  at  the  same 
height  in  both  arms,  and 
there  is  an  equilibrium. 
The  air  presses  on  the 
mercury  in  the  long  arm 
with  a  force  equal  to  a 
column  of  mercury  30 
inches  high,  and  the  elas- 
tic force  of  the  air  con- 
fined in  the  short  arm  is 
equal  to  the  same  amount. 
Now  pour  additional  mer- 
cury into  the  long  arm 
until  it  stands  at  30 
inches  above  that  in  the  Marietta  Tube, 

short  arm  (Fig.  105),  and  the  pressure  is  doubled.    In 
the  short  arm,  the  air  is  condensed    to  one  half   its 


*  This  law  was  independently  discovered  by  the  Englishman,  Boyle, 
and  the  Frenchman,  Mariotte,  during  the  latter  part  of  the  seventeenth 
century.  It  is  often  called  Boyle's  Law. 

t  By  cautiously  inclining  the  apparatus,  when  a  little  air  will  escape, 
and  adding  more  mercury  if  needed,  the  liquid  can  be  made  to  stand  at 
zero  in  both  arms. 


138         PRESSURE     OF     LIQUIDS     AND     GASES. 

FIG.  106.  former  dimensions,  and  the  elastic  force 
is  also  doubled.*  We  therefore  conclude 
that  the  elasticity  of  a  gas  increases,  and 
the  volume  diminishes  in  proportion  to 
the  pressure  upon  it. 

(7.)  The  BAROMETER  is  an  instrument 
for  measuring  the  pressure  of  the  air.  It 
consists  essentially  of  the  tube  and  cup 
of  mercury  in  Fig.  103.  A  scale  is  at- 
tached for  convenience  of  reference.  The 
barometer  is  used  (a)  to  indicate  the 
weather,  and  (&)  to  measure  the  height  of 
mountains. 

It  does  not  directly  foretell  the  weather. 
It  simply  shows  the  varying  pressure  of 
the  air,  from  which  we  must  draw  our 
conclusions.  A  continued  rise  of  the  mer- 
cury indicates  fair  weather,  and  a  con- 
tinued fall,  foul  weather,  f  Since  the  press- 

*  The  force  with  which  the  flying  molecules  of  air 
beat  against  the  walls  of  any  confining  vessel  will  increase 
with  the  diminution  of  the  space  through  which  they  can 
pass.  If  we  give  them  only  half  the  distance  to  fly 
through,  they  will  strike  twice  as  often  and  exert  twice 
the  pressure. 

t  Mercury  is  used  for  filling  the  barometer  because  of 
its  weight  and  low  freezing-point.  It  is  said  that  the  first 
barometer  was  filled  with  water.  The  inventor,  Otto  von 
Guericke,  erected  a  tall  tube  reaching  from  a  cistern  in  the 
cellar  up  through  the  roof  of  his  house.  A  wooden  image 
was  placed  within  the  tube,  floating  upon  the  water.  On 
fine  days,  this  noVel  weather-prophet  would  rise  above  the 
roof-top  and  peep  out  upon  the  queer  old  gables  of  that 
ancient  city,  while  in  foul  weather  he  would  retire  to  the 
protection  of  the  garret.  The  accuracy  of  these  movements 
The  Barometer,  attracted  the  attention  of  the  neighbors.  Finally,  becoming 


PNEUMATICS. 


139 


ure  diminishes  above  the  level  of  the  sea,  the  ob- 
server ascertains  the  fall  of  the  mercury  in  the 
barometer,  and  the  temperature  by  the  thermometer ; 
and  then,  by  reference  to  tables,  determines  the 
height. 


PIG.  107. 


FIG.  108. 


FIG.  109. 


The  Lifting-pump. 

5.  Pumps. — (1.)  The  LIFTING-PUMP  contains  two 
valves  opening  upward — one,  a,  at  the  top  of  the 
suction-pipe,  B ;  the  other,  c,  in  the  piston.  Suppose 
the  handle  to  be  raised,  the  piston  being  at  the  bot- 
tom of  the  cylinder  and  both  valves  closed.  Now 

suspicious  of  Otto's  piety,  they  accused  him  of  being  in  league  with  the 
devil.  So  the  offending  philosopher  relieved  this  wicked  wooden  man  from 
longer  dancing  attendance  upon  the  weather,  and  the  staid  old  city  was 
once  more  at  peace. 


140        PRESSURE     OF     LIQUIDS     AND     GASES. 


Fio.  110. 


depress  the  pump-handle  and  thereby  elevate  the 
piston.  This  will  produce  a  partial  vacuum  in  the 
suction-pipe.  The  pressure  of  the  air  on  the  surface 
of  the  water  below  will  force  the  water  up  the  pipe, 
open  the  valve,  and  partly  fill  the 
chamber.  Let  the  pump-handle  be 
elevated  again,  and  the  piston  de- 
pressed. The  valve  a  will  then  close, 
the  valve  c  will  open,  and  the  water 
will  rise  above  the  piston  (Fig.  108). 
When  the  pump-handle  is  lowered 
the  second  time  and  the  piston  ele- 
vated, the  water  is  lifted  up  to  the 
spout,  whence  it  flows  out ;  while  at 
the  same  time  the  lower  valve  opens 
and  the  water  is  forced  up  from  be- 
low by  the  pressure  of  the  air  (Fig. 
109).* 

(2.)  The  FORCE-PUMP  has  no  valve 
in  the  piston.  The  water  rises  above 
the  lower  valve  as  in  the  lifting-pump. 
When  the  piston  descends,  -the  press- 
ure opens  the  valve  in  the  pipe  D,  and  forces  the 
water  up.  This  pipe  may  be  made  of  any  length, 
and  thus  the  water  driven  to  any  height. 

(3.)  The  FIRE-ENGINE  consists  of  two  force-pumps 
with  an  air-chamber.  The  water  is  driven  by  the 

*  If  the  valves  and  piston  were  fitted  air-tight,  the  water  could  be 
raised  34  feet  (more  exactly  13£  times  the  height  of  the  barometric  column) 
to  the  lower  valve,  but  owing  to  various  imperfections  it  commonly  reaches 
about  28  feet.  For  a  similar  reason  we  sometimes  find  a  dozen  strokes 
necessary  to  "bring  water." 


Force-pump. 


PNEUMATICS. 


141 


FIG.  111. 


Fire-engine. 


pistons  ra,  n,  alter- 
nately, into  the 
chamber  R,  whence 
the  air,  by  its  ex- 
pansive force,  throws 
it  out  in  a  continu- 
ous stream  through 
the  hose-pipe  at- 
tached at  Z  (Fig.  1 1 1 ). 

6.    The 

Siphon  is  a 
U-shaped 
tube,  hav- 
ing one  arm 
longer  than 


FIG.  112. 


siphon. 


142         PRESSURE     OF     LIQUIDS     AND     GASES. 

the  other.  Insert  the  short  arm  in  the  water,  and 
then  applying  the  mouth  to  the  long  arm,  exhaust 
the  air.  The  water  will  flow  from  the  long  arm 
until  the  end  of  the  short  arm  is  uncovered. 

THEORY  OF  THE  SIPHON. — The  pressure  of  the  air 
at  5  holds  up  the  column  of  water  a  &,  and  the  up- 
ward pressure  is  the  weight  of  the  air  less  the  weight 
of  the  column  of  water  a  Z>.  The  upward  pressure 
at  d  is  the  weight  of  the  air  minus  the  weight  of 
the  column  of  water  c  d.  Now  c  d  is  less  than  a  &, 
and  the  water  in  the  tube  is  driven  toward  the 
longer  arm  by  a  force  equal  to  the  difference  in  the 

weight    of   the    two    arms.* 

FIG.  113. 

7.  The  Pneumatic  Inkstand 

can  be  filled  only  when  tipped 
so  that  the  nozzle  is  at  the 
top.  The  pressure  of  the  air 
will  retain  the  ink  when  the 
stand  is  placed  upright.  When 

Pneumatic  Inkstand.  nil  i-iii          j?       • 

used  below  o,  a  bubble  of  air 
passes  in,  forcing  the  ink  into  the  nozzle. 

8.  The  Hydraulic  Ram  is  a  machine  for  raising 
water  where  there  is  a  slight  fall.  The  water  enters 

*  The  siphon  is  used  more  conveniently  if  two  tubes  of  glass  or  metal 
are  connected  with  a  flexible  tube  of  India  rubber.  An  instructive  experi- 
ment can  then  be  made  if  we  allow  the  water  to  run  from  one  tumbler 
into  another  until  just  before  the  flow  ceases;  then  quickly  elevate  the 
glass  containing  the  long  arm,  carefully  keeping  both  ends  of  the  siphon 
under  the  water,  when  the  flow  will  set  back  to  the  first  tumbler.  Thus 
we  may  alternate  until  we  see  that  the  water  flows  to  the  lower  level,  and 
ceases  whenever  it  reaches  the  same  level  in  both  glasses.  It  will  add  to 
the  beauty  of  this  as  weU  as  of  many  other  experiments,  to  color  the  water 
in  one  tumbler  with  a  few  scales  of  magenta,  or  with  red  ink. 


PNEUMATICS. 


143 


through  the  pipe  A,   fills  the  reservoir  B,  and  lifts 
the  valve  D.     As  that  closes,  the  shock  raises  the 


PIG.  115. 


Hydraulic  Ram. 

valve  E  and  drives  the  water  into  the  air-chamber 
G.     D  falls   again   as  soon  as  an  equilibrium  is  re- 
stored.     A  second  shock  follows,  and  more  water  is 
thrown  into  G.     When  the 
air  in  G-  is  sufficiently  con- 
densed,    its     elastic     force 
drives    the    water    through 
the  pipe  H. 

9.  The  Atomizer  is  used 
to  turn  a  liquid  into  spray. 
The  blast  of  air  driven  from 
the  rubber  bulb  as  it  passes 
over  the  end  of  the  upright 
tube,  sweeps  along  the 
neighboring  molecules  of  air 
and  produces  a  partial  vacuum  in  the  tube.*  The 


Atomizer. 


*  In  locomotives,  this  principle  of  the  adhesion  of  gases  to  gases  is  ap- 
plied to  produce  a  draft.     The  waste  steam  is  thrown  into  the  smoke-pipe, 


14.4        PRESSURE     OF     LIQUIDS     AND     GASES. 

pressure  of  the  air  in  the  bottle  drives  the  liquid  up 
the  tube,  and  at  the  mouth  the  blast  of  air  carries 
it  off  in  fine  drops. 

The  action  of  a  current  of  air  in  dragging  along 
with  it  the  adjacent  still  atmosphere  and  so  tending 
to  produce  a  vacuum,  is  shown  by  the  apparatus 
represented  in  Fig.  116.  A  globe,  a,  is  connected 

FIG.  116. 


Tube  for  showing  Adhesion  of  Air. 


with  a  horizontal  tube,  c,  containing  colored  water. 
Close  the  opening  d  with  the  finger,  and  with  the 
mouth  at  &  draw  the  air  out  of  the  globe.  A  slight 
rarefaction  will  cause  the  liquid,  by  the  pressure  of 
the  air  at  the  opening  /,  to  be  forced  into  a.  Now, 

and  this  current  sweeps  off  the  smoke  from  the  fire,  while  the  pressure  of 
the  atmosphere  outside  forces  the  air  through  the  furnace  and  increases 
the  combustion.— A  familiar  illustration  may  be  devised  by  taking  two 
disks  of  card-board,  the  lower  one  fitted  with  a  quill,  and  the  upper  one 
merely  kept  from  sliding  off  by  a  pin  thrust  through  it  and  extending  into 
the  quill.  The  more  forcibly  air  is  driven  through  the  quill  against  the 
upper  disk,  the  more  firmly  it  will  be  held  to  its  place.  See  article  "  Ball 
Paradox,"  in  "Popular  Science  Monthly,"  April,  1877.— Faraday  used  to 
illustrate  the  principle  thus:  Hold  the  hand  out  flat  with  the  fingers 
extended  and  pressed  together.  Place  underneath  a  piece  of  paper  two 
inches  square.  Blow  through  the  opening  between  the  index  and  the 
middle  finger,  and  so  long  as  the  current  is  passing  the  paper  will  not 
fall. 


PKACTICAL     QUESTIONS.  145 

if,  instead  of  drawing  the  air  out  at  &,  a  jet  of  air 
be  forced  through  the  tube  and  out  at  d,  the  same 
effect  will  be  produced. 

1C.  Height  of  the  Atmosphere. — Three  opposing 
forces  act  upon  the  air,  viz.:  gravity,  which  binds  it 
to  the  earth,  and  the  centrifugal  and  repellent  forces, 
which  tend  to  hurl  it  into  space.  There  must  be  a 
point  where  these  balance.  At  the  height  of  3.4 
miles  the  mercury  in  the  barometer  stands  at  15 
inches,  indicating  that  half  the  atmosphere  is  within 
about  3^  miles  of  the  earth's  surface.  Beyond  a 
height  of  40  miles  the  quantity  of  air  is  too  small 
to  be  perceptible  in  any  way.*  If  it  were  every-where 
as  dense  as  it  is  at  sea-level,  the  upper  limit  of  our 
atmosphere  would  be  about  five  miles  high. 


PRACTICAL     QUESTIONS. 

[In  these  questions,  assume  the  standard  conditions  mentioned  on  p.  1S6."] 

1.  Why  must  we  make  two  openings  in  a  barrel  of  cider  when  we 
tap  it? 

2.  What  is  the  weight  of  10  cubic  feet  of  air? 

3.  What  is  the  pressure  of  the  air  on  1  square  rod  of  land? 

4.  What  is  the  pressure  on  a  pair  of  Magdeburg  hemispheres  4  inches 
in  diameter? 

5.  How  high  a  column  of  water  can  the  air  sustain  when  the  barometric 
column  stands  at  28  inches? 

6.  If  we   should   add  a  pressure  of  two   atmospheres   (30   Ibs.  to  the 
square  inch),  what  would  be  the  volume  of  100  cubic  inches  of  common 
air? 


*  In  mountain  climbing,  or  ascending  to  a  great  height  in  a  balloon, 
the  voyager  is  apt  to  suffer  on  account  of  the  decrease  in  density  of  the 
air.  In  1862,  Mr.  G-laisher  ascended  nearly  7  miles,  and  there  fainted. 
His  assistant  was  barely  able  to  open  the  valve  and  cause  the  balloon  to 
descend. 


146         PRESSURE     OF     LIQUIDS     AND     GASES. 

7.  If,  while  the  water  is  running  through  the  siphon,  we  quickly  lift 
the  long  arm,  what  is  the  effect  on  the  water  in  the  siphon  ?    If  we  lift  the 
entire  siphon? 

8.  When  the  mercury  stands  at  29$  inches  in  the  barometer,  how  high 
above  the  surface  of  the  water  can  we  place  the  lower  pump- valve? 

9.  Can  we  raise  water  to  a  higher  level  by  means  of  a  siphon? 

10.  If  the  air  in  the  chamber  of  a  fire-engine  be  condensed  to  TV  its 
former  bulk,  what  will  be  the  pressure  due  to  the  expansive  force  of  the 
air  on  every  square  inch  of  the  air-chamber? 

11.  What  causes  the  bubbles  to  rise  to  the  surface  when  we  put  a  lump 
of  loaf-sugar  in  hot  tea? 

12.  When  will  a  baUoon  stop  rising?   What  weight  can  it  lift? 

13.  The  rise  and  fall  of  the  barometric  column  shows  that  the  air  is 
lighter  in  foul  and  heavier  in  fair  weather.     Why  is  this?     Ans.  Vapor  of 
water  is  only  half  as  heavy  as  dry  air.     When  there  is  a  large  quantity 
present  in  the  atmosphere,  displacing  its  own  volume  of  air,  the  weight  of 
the  atmosphere  will  be  correspondingly  diminished. 

14.  When  smoke  ascends  in  a  straight  line  from  chimneys,  is  it  a  proof 
of  the  rarity  or  the  density  of  the  air? 

15.  Explain  the  action  of  the  common  leather-sucker. 

16.  Did  you  ever  see  a  bottle  really  empty? 

17.  Why  is  it  so  tiresome  to  walk  in  miry  clay  ?     Ans.   Because  the  up- 
ward pressure  of  the  air  is  removed  from  our  feet. 

18.  How  does  the  variation  in  the  pressure  of  the  air  affect  those  who 
ascend  lofty  mountains?    Who  descend  in  diving-bells? 

19.  Explain  the  theory  of  "sucking  cider"  through  a  straw. 

20.  Would  it  make  any  difference  in  the  action  of  the  siphon  if  the 
limbs  were  of  unequal  diameter? 

21.  What  would  be  the  effect  of  making  a  small  hole  in  the  top  of  a 
diving-bell  while  in  use? 

22.  The  pressure  of  the  atmosphere  being  1.03  kg.  per  sq.  cm.,  what  is 
the  amount  on  10  sq.  meters? 


SUMMARY. 

Hydrostatics  treats  of  the  laws  of  equilibrium  in  liquids. 
Pressure  is  transmitted  by  liquids  equally  in  every  direction. 
Water  thus  becomes  a  "mechanical  power,"  as  in  the  "Hy- 
draulic Press."  Liquids  acted  on  by  their  weight  only,  at  the 
same  depth,  press  downward,  upward,  and  sidewise  with  equal 
force.  This  pressure  is  independent  of  the  size  of  the  vessel, 
but  increases  with  the  depth.  Wells,  springs,  aqueducts,  fount- 
ains, and  the  water-supply  of  cities  illustrate  the  tendency  of 


SUMMARY.  147 

water  to  seek  its  level.  The  ancients  understood  this  law,  but 
had  no  suitable  material  for  making  the  immense  pipes  needed ; 
just  so  the  art  of  printing  awaited  the  invention  of  paper.  Spe- 
cific gravity,  or  the  relative  weights  of  the  same  volume  of  dif- 
ferent substances,  is  found  by  comparing  them  with  the  weight 
of  the  same  volume  of  water.  This  is  easily  done,  since,  according 
to  the  law  of  Archimedes,  a  body  immersed  in  water  is  buoyed 
up  by  a  force  equal  to  the  weight  of  the  water  displaced ;  i.  e., 
it  loses  in  weight  an  amount  equal  to  that  of  the  same  volume  of 

weight  in  vacuum 

water.     Hence  spec.  grav.= - — : 

weight  in  vacuum  —  weight  in  water 

A  floating  body  displaces  only  its  own  weight  of  liquid.  This  ex- 
plains the  buoyancy  which  supports  a  ship,  why  a  floating  log 
is  partly  out  of  water,  and  many  similar  phenomena. 

Hydrodynamics  treats  of  moving  liquids.  The  laws  of  falling 
bodies  in  theory  apply ;  so  that  a  descending  jet  of  water 
will  acquire  the  same  velocity  that  a  stone  would  in  falling  to 
the  ground  from  the  surface  of  the  water;  and  an  ascending 
jet  would  need  to  have  the  same  velocity  in  order  to  reach  that 
height.  The  quantity  of  water  discharged  through  any  orifice 
equals  the  area  of  the  opening  multiplied  by  the  velocity  of  the 
stream.  The  chief  resistance  to  the  motion  of  a  liquid  is  the 
friction  of  the  air  and  against  the  sides  of  the  pipe,  and,  in  the 
case  of  rivers,  against  the  banks  and  bottom  of  the  channel. 
The  force, of  falling  water  is  utilized  in  the  arts  by  means  of 
water-whee]s.  There  are  four  kinds — overshot,  undershot, 
breast,  and  turbine.  The  principles  of  wave-motion,  so  essen- 
tial to  the  understanding  of  sound,  light,  etc.,  are  most  easily 
studied  in  connection  with  water.  A  stone  let  fall  into  a  quiet 
pool  sets  in  motion  a  series  of  concentric*  waves,  whose  particles 
move  in  ellipses,  while  the  movement  passes  to  the  outermost 
edge  of  the  water,  and  is  then  transmitted  to  the  ground  be- 
yond. The  velocity  of  the  particles  is  much  less  than  that  of 
the  wave  itself.  A  handful  of  stones  acts  in  the  same  way,  but 
sets  in  motion  many  series  of  waves.  Hence  arise  the  phenom- 
ena of  interference. 

Pneumatics  treats  of  the  properties  and  the  laws,  of  equilib- 
rium of  gases.  The  air  being  composed  of  matter,  has  all  the 
properties  we  associate  with  matter,  as  weight,  indestructibility, 


148        PRESSURE     OF     LIQUIDS     AND     GASES. 

extension,  compressibility,  etc.  The  elasticity  of  the  air,  accord- 
ing to  Mariotte's  law,  is  inversely  proportional  to  its  volume,  and 
this  is  inversely  proportional  to  the  pressure  upon  the  air ;  both 
heat  and  pressure  increasing  the  elasticity  of  a  gas.  The  air,  like 
other  fluids,  transmits  the  weight  of  its  own  particles,  as  well 
as  any  outside  pressure,  equally  in  every  direction ;  hence  the 
upward  pressure  or  buoyant  force  of  the  atmosphere.  A  balloon 
rises  because  it  is  buoyed  up  by  a  force  equal  to  the  weight  of 
the  air  it  displaces.  It  floats  in  the  air  for  the  same  reason 
that  a  ship  floats  on  the  ocean.  When  smoke  falls  it  is  heavier 
than  the  surrounding  atmosphere.  When  it  rises,  it  is  carried  up 
by  adhesion  of  warm  air,  which  is  lighter  than  that  surrounding 
the  current.  The  air-pump  is  used  for  exhausting  the  air  from, 
and  the  condenser  for  condensing  the  air  into,  a  receiver.  A 
vacuum  in  which  there  remains  only  i  00^  OTrg  of  the  atmosphere 
can  be  obtained  by  means  of  Sprengel's  air-pump,  which  acts  on 
the  principle  of  the  adhesion  of  the  air  to  a  column  of  falling 
mercury.  The  average  pressure  of  the  air  being  15  Ibs.  to  the 
square  inch,  equals  that  of  a  column  of  water  34  feet,  and  of 
mercury  30  inches  or  760  millimeters  high.  This  amount  varies 
incessantly  through  atmospheric  changes  caused  by  alterations 
in  the  wind,  heat  of  the  sun,  etc.  The  barometer  measures  the 
pressure  of  the  atmosphere,  and  is  used  to  determine  the  height 
of  mountains  and  the  changes  of  the  weather.  The  action  of 
the  siphon,  the  pneumatic  inkstand,  and  of  the  different  kinds 
of  pumps,  is  based  upon  the  pressure  of  the  air. 


HISTORICAL     SKETCH. 

Hydrostatics  is  comparatively  a  modern  science.  The  Ro- 
mans had  a  knowledge  of  the  fact  that  "  liquids  rise  to  the 
level  of  their  source,"  but  they  had  no  means  of  making  iron 
pipes  strong  enough  to  resist  the  pressure.*  They  were  there- 

*  The  ancient  engineers  sometimes  availed  themselves  of  this  principle. 
Not  far  from  Rachel's  Tomb,  Jerusalem,  are  the  remains  of  a  conduit  once 
used  for  supplying  the  city  with  water.  The  valley  was  crossed  by  means 
of  an  inverted  siphon.  The  pipe  was  about  two  miles  long  and  fifteen 


HISTORICAL • SKETCH.  149 

fore  forced  to  carry  water  into  the  Imperial  City  by  means  of 
enormous  aqueducts,  one  of  which  was  63  miles  long,  and  was 
supported  by  arches  100  feet  high.  The  ancient  Egyptians  and 
Chaldeans  were  probably  the  first  to  investigate  the  most  obvi- 
ous laws  of  liquids  from  the  necessity  of  irrigating  their  land. 
Archimedes,  in  the  third  century  B.C.,  invented  a  kind  of  pump 
called  Archimedes'  Screw,  demonstrated  the  principle  of  equilib- 
rium, known  now  as  "Archimedes'  Law"  (p.  114),  and  found 
out  the  method  of  obtaining  the  specific  gravity  of  bodies.  The 
discovery  of  the  last  is  historical.  Hiero  of  Syracuse  suspected 
that  a  gold  crown  had  been  fraudulently  alloyed  with  silver. 
He  accordingly  asked  Archimedes  to  find  out  the  fact  without 
injuring  the  workmanship  of  the  crown.  One  day  going  into  a 
bath-tub  full  of  water,  the  thought  struck  the  philosopher  that 
as  much  water  must  run  over  the  side  as  was  equal  to  the 
volume  of  his  body.  Electrified  by  the  idea,  he  sprang  out  and 
ran  through  the  streets,  shouting :  "  Eureka ! "  (I  have  found  it !) 
The  ancients  never  dreamed  of  associating  the  air  with  gross 
matter.  To  them  it  was  the  spirit,  the  life,  the  breath.  No- 
ticing how  the  atmosphere  rushes  in  to  fill  any  vacant  space, 
the  followers  of  Aristotle  explained  it  by  saying,  "Nature 
abhors  a  vacuum."  This  principle  answered  the  purpose  of 
philosophers  for  2,000  years.  In  1640,  some  workmen  were 
employed  by  the  Duke  of  Tuscany  to  dig  a  deep  well  near 
Florence.  They  found  to  their  surprise  that  the  water  would 
not  rise  in  the  pump  as  high  as  the  lower  valve.  More  disgusted 
with  nature  than  nature  was  with  the  vacuum  in  their  pump, 
they  applied  to  Q-alileo.  The  aged  philosopher  answered — half 
in  jest,  we  hope,  certainly  he  was  half  in  earnest — "Nature  does 
not  abhor  a  vacuum  beyond  34  feet."  His  pupil,  Torricelli,  how- 
ever, discovered  the  secret.  He  reasoned  that  there  is  a  force 
which  holds  up  the  water,  and  as  mercury  is  13£  times  as 
heavy  as  water,  it  would  sustain  a  column  of  that  liquid  only 
34  feet  -f- 13£  =  30  inches  high.  Trying  the  experiment  shown  in 
Fig.  103,  he  verified  the  conclusion  that  the  weight  of  the  air  is 
the  unknown  force.  But  the  opinion  was  not  generally  received. 

inches  in  diameter.      It   consisted  of  perforated   blocks  of  stone,  ground 
smooth  at  the  joints,  and  fastened  with  a  hard  cement. 


150        PRESSURE     OF     LIQUIDS     AND     GASES. 

Pascal  next  reasoned  that  if  the  weight  of  the  air  is  really  the 
force,  then  at  the  summit  of  a  high  mountain  it  is  weakened, 
and  the  column  would  be  lower.  He  accordingly  carried  his 
apparatus  to  the  top  of  a  tower,  and  finding  a  slight  fall  in  the 
mercury,  he  asked  his  brother-in-law,  Perrier,  who  lived  near 
Puy  de  D6me,  a  mountain  in  Southern  France,  to  test  the  con- 
clusion. On  trial,  it  was  found  that  the  mercury  fell  3  inches. 
"A  result,"  wrote  Perrier,  "which  ravished  us  with  admiration 
and  astonishment."  Thus  was  discovered  the  germ  of  our  mod- 
ern barometer,  and  the  dogma  of  the  philosophers  soon  gave 
place  to  the  law  of  gravitation  and  our  present  views  concern- 
ing the  atmosphere. 

Consult  Pepper's  "  Cyclopedic  Science";  Bert's  "Atmos- 
pheric Pressure  and  Life,"  in  "Popular  Science  Monthly,"  Vol. 
XI.,  p.  316;  "Appleton's  Cyclopedia,"  Articles  on  Hydrome- 
chanics, Atmosphere,  Pneumatics,  etc.;  Delaunay,  "  Me"canique 
JRationelle " ;  Boutan  et  D' Almeida,  "Cours  de  Physique"; 
Miiller,  "Lehrbuch  der  Physik  und  Meteorologie." 

On  the  theory  of  Wave-motion,  and  the  subjects  of  Sound 
and  Light,  which  are  now  to  follow,  consult  Lockyer's  "  Studies 
in  Spectrum  Analysis";  Lloyd's  "Wave  Theory";  Taylor's 
"Science  of  Music";  Blaserna's  "Theory  of  Sound  in  Relation 
to  Music";  Tyndall's  "Sound"  and  "Light";  Lockyer's  "  Water- 
waves  and  Sound-waves"  in  "Popular  Science  Monthly,"  Vol. 
XIII.,  p.  166;  Shaw's  "How  Sound  and  Words  are  Produced," 
in  "Popular  Science  Monthly,"  Vol.  XIII.,  p.  43;  Mayer  on 
"Sound";  Schellen's  "Spectrum  Analysis";  Airy's  "Optics"; 
Lockyer's  "Spectroscope";  Chevreul's  "Colors";  Spottiswoode's 
"Polarization  of  Light";  Lommel's  "Nature  of  Light";  Helm- 
holtz's  "Popular  Lectures  on  Scientific  Subjects";  "Appleton's 
Cyclopedia,"  Articles  on  Sound,  Light,  Spectrum,  Spectrum  Anal- 
ysis, Spectacles,  Heat,  etc. ;  Stokes'  "Absorption  and  Colors,"  and 
Forbes'  "Radiation,"  in  "Science  Lectures  at  South  Kensington," 
Vol.  I.;  Mayer  and  Barnard's  "Light";  Draper's  "Popular  Ex- 
position of  some  Scientific  Experiments,"  in  "Harper's  Maga- 
zine" for  1877;  Core's  "Modern  Discoveries  in  Sound,"  in 
Manchester  Science  Lectures,  '77-8 ;  Dolbear's  "  Art  of  Project- 
ing"; Draper's  "Scientific  Memoirs";  -Steele's  "Physiology," 
Section  on  Sight,  pp.  187-196. 


VI. 

ON  SOUND. 


SCIENCE  ought  to  teach  us  to  see  the  invisible  as  well  as  the  visible  in 
nature :  to  picture  to  our  mind's  eye  those  operations  that  entirely  elude 
the  eye  of  the  body ;  to  look  at  the  very  atoms  of  matter,  in  motion  and  in 
rest,  and  to  follow  them  forth  into  the  world  of  the  senses."— TYNDALL. 


ANALYSIS  OF  SOUND. 


1.  PRODUCTION  OF  SOUND. 


2.  TRANSMISSION  OF  SOUND. 


3.  REFRACTION  OF  SOUND. 


4.  REFLECTION  OF  SOUND. 


5.  MUSICAL  SOUNDS. 


6.  INTERFERENCE  OF  SOUND. 


7.  VIBRATION  OF  CORDS. 


(1.)  Through  Air. 
(2.)  In  a  Vacuum. 
(3.)  In  Liquids. 
(4.)  In  Solids. 

(5.)  Production  of  Motion  by  Sound. 
(6.)  Co-vibration   through  Air  as   a  Me- 
dium. 

(7.)  Velocity  of  Transmission. 
.  (8.)  Loudness  of  Sound. 

!(1.)  Law  of  Reflection. 
(2.)  Echoes. 
(3.)  Decrease  by  Reflection. 
(4.)  Acoustic  Clouds. 

f  (1.)  Difference  between  Noise  and  Music. 

(2.)  Pitch. 

(3.)  The  Siren. 

(4.)  Wave-lengths. 

(5.)  Tones  in  Unison. 

(1.)  The  Sonometer. 

(2.)  Laws  of  Vibration. 

(3.)  Nodes. 

(4.)  Acoustic  Figures. 

(5.)  Harmonics. 

(6.)  Nodes  of  a  Bell. 

(7.)  Nodes  of  a  Sounding-board. 

L  (8.)  Musical  Scale. 


8.  VIBRATION  OF  COLUMNS  OF  AIR. 


9.  "Wnn>  INSTRUMENTS. 

10.  CO-VIBRATION. 

11.  THE  PHONOGRAPH. 

12.  THE  BAR. 


(1.)  Sensitive  Flames. 
(2.)  Singing  Flames. 


(1.)  Range  of  the  Ear. 

(2.)  Ability  to  Analyze  Sound 


ACOUSTICS,  OR  THE  SCIENCE  OF 
SOUND.* 

1.  Production  of  Sound.— By  lightly  tapping  a 
glass  fruit-dish,  we  can  throw  the  sides  into  motion 
visible  to  the  eye. — Fill  a  goblet  half-full  of  water, 
and  rub  a  wet  finger  lightly  around  the  upper  edge 
of  the  glass.  The  sides  will  vibrate,  and  cause  tiny 
waves  to  ripple  the  surface  of  the  water. — Hold  a 
card  close  to  the  prongs  of  a  vibrating 
tuning-fork,  and  you 
can  hear  the  repeat- 
ed taps. 
Place 

the  cheek  near  them,     *"•****  *«tatata« its  vibrations- 
and  you  will  feel  the  little  puffs  of  wind.    Insert  the 
handle  between  your  teeth,  and  you  will  experience 
the  indescribable  thrill  of  the  swinging  metal.     The 

*  The  term  sound  is  used  in  two  senses— the  subjective  (which  has  refer- 
ence to  our  mind)  and  the  objective  (which  refers  to  the  objects  around  us). 
(1.)  Sound  is  the  sensation  produced  upon  the  organ  of  hearing  by  vibra- 
tions in  matter.  In  this  use  of  the  word  there  can  be  no  sound  where 
there  is  no  ear  to  catch  the  vibrations.— An  oak  falls  in  the  forest,  and  if 
there  is  no  ear  to  hear  it  there  is  no  noise,  and  the  old  tree  drops  quietly 
to  its  resting-place.— Niagara's  flood  poured  over  its  rocky  precipice  for 
ages,  but  fell  silently  to  the  ground.  There  were  the  vibrations  of  earth 
and  air,  but  there  was  no  ear  to  receive  them  and  translate  them  into 
sound.  When  the  first  foot  trod  the  primeval  solitude,  and  the  ear  felt  the 
pulsations  from  the  torrent,  then  the  roaring  cataract  found  a  voice  and 
broke  its  lasting  silence.  (2.)  Sound  consists  of  those  vibrations  of  matter 


154 


ACOUSTICS. 


tuning-fork  may  be  made  to  draw  the  outline  of  its 
vibrations  upon  a  smoked  glass.  Fasten  upon  one 
prong  a  sharp  point,  and  drawing  the  fork  along,  a 
sinuous  line  will  show  the  width  (amplitude)  of  the 
vibrations. 

2.  Transmission  of  Sound.  (1.)  THROUGH  AIR. 
In  order  that  any  medium  shall  transmit  sound,  it 
must  be  elastic.  Most  known  bodies  possess  some 
elasticity,  and  hence  sound  may  be  transmitted 
through  gases,  liquids,  and  solids.  The  prong  of  a 
tuning-fork  advances,  condensing  the  elastic  air  in 
front  of  it.  This  transmits  the  compression  to  the 
air  next  forward,  while  the  fork  swings  backward, 
leaving  a  rarefaction  next  to  the  compression.  This 


in  turn  advances,  on  account  of  the  elasticity 
of  the  air,  and  is  followed  by  a  compression 
due  to  the  second  forward  swing  of  the  fork. 
This  process  is  repeated,  until  the  fork  comes  to  rest, 
and  the  sound  ceases.  Each  vibration  produces  a 
sowid^wave  of  air,  which  contains  one  condensation 
and  one  rarefaction.  In  water,  we  measure  a  wave- 
capable  of  producing  a  sensation  upon  the  organ  of  hearing.  In  this  use 
of  the  word  there  can  be  a  sound  in  the  absence  of  the  ear.  An  object 
falls  and  the  vibrations  are  produced,  though  there  may  be  no  organ  of 
hearing  to  receive  an  impression  from  them.  This  is  the  sense  in  which 
the  term  sound  is  used  in  Physics. 


TRANSMISSION     OF     SOUND. 


165 


length  from  crest  to  crest ;  in  air,  from  condensation 
to  condensation.  The  condensation  of  the  sound- 
wave corresponds  to  the  crest,  and  the  rarefaction  of 
the  sound-wave  to  the  hollow  of  the  water-wave.  In 
Fig.  118,  the  dark  spaces  a,  5,  c,  d  represent  the  con- 
densations, and  a',  5',  c'  the  rarefactions;  the  wave- 
lengths are  the  distances  a&,  &c,  cd. 

If  we  fire  a  gwi,  the  gases  which  are  produced 
expand  suddenly  and  force  the  air  outward  in  every 

Fio.  119. 


•    alii 


Propagation  of  Sound. 

direction.  This  hollow  shell  of  condensed  air  imparts 
its  motion  to  the  next  one,  while  it  springs  back  by 
its  elasticity  and  becomes  rarefied.  The  second  shell 
rushes  forward  with  the  motion  received,  then 
bounds  back  and  becomes  rarefied.  Thus  each  shell 
of  air  takes  up  the  motion  and  imparts  it  to  the 
next.  The  wave,  consisting  of  a  condensation  and  a 
rarefaction,  proceeds  onward.  It  is,  however,  as  in 
water-waves,  a  movement  of  the  form  only,  while 
the  particles  vibrate  but  a  short  distance  to  and  fro. 


156 


ACOUSTICS. 


FIG.  130. 


The    molecules   in   water-waves   oscillate    vertically; 

those  in  sound-waves  horizontally,  or  parallel  to  the 

line  of  motion.* 

If  a  bell  be  rung,  the  adja- 
cent air  is  set  in  motion ; 
thence,  by  a  series  of  conden- 
sations and  rarefactions,  the 
vibrations  are  conveyed  to  the 
ear.f 

(2.)  IN  A  VACUUM.  The  bell 
B  (Fig.  120)  may  be  set  in 
motion  by  the  sliding-rod  r. 
The  apparatus  is  suspended 
by  silk  cords,  that  no  vibration 
may  be  conducted  through  the 
pump.  If  the  air  be  exhausted, 
the  sound  will  become  so  faint 
that  it  can  not  be  heard,  except 
when  the  ear  is  placed  close  to 
the  receiver.]: 

In    very    elevated     regions 

sounds  are  diminished  in  loudness,  and  it  is  difficult 


Bell  in  Vacuum. 


*  A  continuous  blast  of  air  produces  no  sound.  The  rush  of  the  grand 
aerial  rivers  above  us  we  never  hear.  They  flow  on  in  the  upper  regions 
ceaselessly  but  silently.  Let,  however,  the  great  billows  strike  a  tree  and 
wrench  it  from  the  ground,  and  we  can  hear  the  secondary,  shorter  waves 
which  set  out  from  the  struggling  limbs  and  the  tossing  leaves. 

t  "  It  is  marvelous,"  says  Youmans,  "  how  slight  an  impulse  throws  a 
vast  amount  of  air  into  motion.  We  can  easily  hear  the  song  of  a  bird 
500  feet  above  us.  For  its  melody  to  reach  us  it  must  have  filled  with 
wave-pulsations  a  sphere  of  air  1,000  feet  in  diameter,  or  set  in  motion 
eighteen  tons  of  the  atmosphere." 

$  There  would  be  perfect  silence  in  a  perfect  vacuum.  ITo  sound  is 
transmitted  to  the  earth  from  the  regions  of  space.  The  movements  of  the 
heavenly  bodies  are  noiseless. 


TKAKSM1SS1ON     OF     SOUND.  157 

to  carry  on  a  conversation.     The  reverse  takes  place 
in  deep  mines  and  diving-bells. 

(3.)  IN  LIQUIDS.  Let  two  persons  immerse  them- 
selves in  water  at  a  distance  of  twenty  or  thirty 
yards  from  one  another.  If  one  of  them  strikes  two 
pebbles  together,  or  rings  a  bell,  the  other  will  hear 
the  sound  with  the  utmost  clearness. 

(4.)  IN  SOLIDS.  The  "Lovers'  Telephone"  consists 
of  a  pair  of  little  cups,  the  bottom  of  each  being 
made  of  an  elastic  substance,  like  stretched  bladder, 
and  connected  with  that  of  the  other  by  a  string. 
By  stretching  the  string  elastic  force  is  developed, 
and  on  talking  into  one  of  the  cups  the  sound  is 
readily  heard  at  the  other.  On  relaxing  the  string 
and  thus  diminishing  the  elasticity,  sound  ceases  to 
be  conveyed  perceptibly  by  it.  By  putting  the  ear 
against  the  ground,  one  may  hear  the  tread  of  foot- 
steps that  are  inaudible  through  the  air  alone.* 

(5.)  PRODUCTION  OF  MOTION  BY  SOUND.  If  a  tuning- 
fork  be  excited  and  its  stem  be  pressed  firmly  against 
a  table,  the  sound  will  become  much  louder.  The 
solid  fork  communicates  its  vibration  to  the  table, 
which  in  turn  gives  its  vibration  to  a  much  larger 
body  of  air  than  that  in  contact  with  the  fork  alone. 
Tuning-forks  are  generally  mounted  upon  resonance 
boxes,  the  whole  body  of  air  within,  as  well  as  the 
box  itself,  thus  co-vibrating  with  the  fork. 

*  Wheatstone  invented  a  beautiful  experiment  to  show  the  transmission 
of  sound  through  wood.  Upon  the  top  of  a  music-box,  he  rested  the  end  of  a 
wooden  rod  reaching  to  the  room  above,  and  insulated  from  the  ceiling  by 
India  rubber.  A  violin  being  placed  on  the  top  of  the  rod,  the  sounds  from 
the  box  below  filled  the  upper  room,  appearing  to  emanate  from  the  violin. 


158 


ACOUSTICS. 


(6.)  CO-VIBRATION  THROUGH  Am  AS  A  MEDIUM.  The 
air  between  any  source  of  sound  and  the  ear  is  like 
an  elastic  spring  between  a  pair  of  tuning-forks  of 
the  same  size  and  material.  When  the  prong  of  the 
first  fork  swings  from  a  to  &  (Fig.  121),  a  condensa- 
tion is  propagated  through  the  spring  and  makes 


yxx 


FIG.  121. 


Tuning-forks  Connected  by  a  Delicate  Wire. 


the  prong  of  the  second  fork  swing  slightly  from  x 
toward  y.  A  rarefaction  follows,  making  it  swing 
from  x  toward  z.  The  succession  of  these  properly- 
timed  impulses  causes  an  accumulation  of  energy  to 
be  imparted  through  the  air  to  the  second  fork, 
which  soon  gives  forth  an  audible  sound.  The  elas- 
tic bodies  composing  the  ear  in  like  manner  accept 
vibrations  from  outside,  and  their  motion  is  perceived 
as  sound. 

(7.)  THE  VELOCITY  OF  SOUND  depends  on  the  ratio 
of  the  elasticity  to  the  density  of  the  medium 
through  which  it  passes.  The  higher  the  elasticity, 


TRANSMISSION     OF     SOUND.  159 

the,  more  promptly  and  rapidly  the  motion  is  trans- 
mittted,  since  the  elastic  force  acts  like  a  bent 
spring  between  the  molecules;  and  the  greater  the 
density,  the  more  molecules  to  be  set  in  motion,  and 
hence  the  slower  the  transmission. 

Sound  travels  through  air  (at  32°  F.)  1,090  feet 
per  second.  A  rise  in  temperature  diminishes  the 
density  of  the  air,  and  thus  increases  the  velocity  of 
sound.  A  difference  of  1°  F.  makes  a  variation  of  a 
little  more  than  one  foot.  Sound  also  moves  faster 
in  damp  than  in  dry  air. 

Sound  travels  through  water  about  ^,700  feet  per 
second.  Water  being  denser  than  air  should  on  this 
account  conduct  sound  more  slowly ;  but  its  high 
elasticity  (p.  10),  measured  by  the  amount  of  force  re- 
quired to  compress  it,  more  than  quadruples  the  rate. 

Sound  travels  through  solids  faster  than  through 
air.  This  may  be  illustrated  by  placing  the  ear  close 
to  the  horizontal  bar  at  one  end  of  an  iron  fence, 
while  a  person  strikes  a  sharp  blow  at  the  other  end. 
Two  sounds  will  reach  the  ear — one  through  the 
metal,  and  afterward  another  through  the  air.  The 
velocity  varies  with  the  nature  of  the  solid.  In  the 
metals  it  is  from  four  to  sixteen  times  that  in  air. 

Different  sounds  travel  with  sensibly  the  same 
velocity*  A  band  may  be  playing  at  a  distance,  yet 

*  It  has  been  said  that  the  "heaviest  thunder  travels  no  faster  than 
the  softest  whisper."  Mallet,  however,  found  that  in  blasting  with  a 
charge  of  2,000  Ibs.,  the  velocity  was  967  feet  per  second,  while  with  12,000 
Ibs.  it  was  increased  to  1,210  feet.  Parry  in  his  Arctic  travels  states  that, 
on  a  certain  occasion,  the  sound  of  the  sunset-gun  reached  his  ears  before 
the  officer's  word  of  command  to  fire,  proving  that  the  report  of  the  can- 
non traveled  sensibly  faster  than  the  sound  of  the  voice. 


160  ACOUSTICS. 

the  harmony  of  the  different  instruments  is  pre- 
served. The  soft  and  the  loud,  the  high  and  the  low 
notes  reach  the  ear  at  the  same  time. 

Velocity  of  sound  used  to  find  distance.  Light 
travels  instantaneously  so  far  as  all  distances  on  the 
earth  are  concerned.  Sound  moves  more  slowly. 
We  see  a  chopper  strike  with  his  ax,  and  a  moment 
elapses  before  we  hear  the  blow.  If  one  second  in- 
tervenes the  distance  is  about  1,090  feet.  By  means 
of  the  second-hand  of  a  watch  or  the  beating  of  our 
pulse,  we  can  count  the  seconds  that  elapse  between 
a  flash  of  lightning  and  the  peal  of  thunder  which 
follows.  Multiplying  the  velocity  of  sound  by  the 
number  of  seconds,  we  obtain  the  distance  of  the 
tfiunder-bolt. 

(8.)  THE  LOUDNESS  OF  SOUND  depends  chiefly  on 
the  amplitude  of  the  vibration,  if  the  air  be  quiet. 
The  energy  of  the  vibration  is  proportional  to  the 
square  of  the  amplitude,  i.  e.,  the  arc  through  which 
the  molecule  swings  to  either  side  of  its  position  of 
rest.  But  loudness  is  a  sensation,  and  no  accurate 
measurement  of  sensations  has  yet  been  made.  The 
loudness  of  sound  depends  also  on  the  density  of  the 
air.  On  the  top  of  a  mountain,  because  of  the  rare 
atmosphere,  there  are  fewer  molecules  to  be  set  in 
motion,  hence  the  effect  on  the  ear  is  less  intense. 

Mechanically  considered,  the  intensity  of  sound* 

*  The  same  proportion  obtains  in  Gravitation,  Sound,  Light,  and  Heat. 
We  have  seen  how  the  motion  of  the  common  Pendulum  is  due  to  the  force 
of  Gravity,  and  reveals  the  Laws  of  Falling  Bodies.  Now  we  find  that  the 
Pendulum,  and  even  the  principles  of  Beflected  Motion  and  Momentum, 
are  linked  with  the  phenomena  of  Sound,  As  we  progress  further,  we  shall 


BEFRACTION     OF     SOUND.  161 

diminishes  as  the  square  of  the  distance  increases. 
The  sound-wave  expands  in  the  form  of  a  sphere. 
The  larger  the  sphere,  the  greater  the  number  of  air 
particles  to  be  set  in  motion,  and  the  feebler  their 
vibration.  The  surfaces  of  spheres  are  proportional 
to  the  squares  of  their  radii ;  the  radii  of  sound- 
spheres  are  their  distances  from  the  center  of  dis- 
turbance. Hence  the  force  with  which  the  molecules 
will  strike  the  ear  decreases  as  the  square  of  our 
distance  from  the  sounding  body  increases. 

Speaking-tubes  conduct  sound  to  distant  rooms 
because  they  prevent  the  waves  from  expanding  and 
losing  their  intensity.*  The  ear-trumpet  collects 
waves  of  sound  and  reflects  them  into  the  ear.  The 
speaking-trumpet  is  based  on  the  same  principle  as 
the  speaking-tube.  The  sound  of  the  voice  is  strength- 
ened also  by  the  co-vibration  of  the  walls  of  the 
trumpet. 

3.  Refraction  of  Sound— When  a  sound-wave 
goes  obliquely  from  one  medium  to  another,  it  is 
bent  out  of  its  course.  Like  light,  it  may  be  passed 
through  a  lens  and  brought  to  a  focus.  In  Fig.  122 
is  shown  a  bag  of  thin  India  rubber  or  collodion,  filled 
with  carbonic  acid  gas  so  as  to  assume  the  form  of 
a  lens.  A  watch  is  placed  at  one  focus  of  this  and 
the  ear  at  the  other.  The  ticks  of  the  watch  can  be 
heard,  while  outside  the  focus  they  are  inaudible. 

find  how  Nature  is  thus  interwoven  every-where  with  proofs  of  a  common 
plan  and  a  common  Author. 

*  Biot  held  a  conversation  through  a  Paris  water-pipe  3,120  feet  long. 
He  says  that  "  it  was  so  easy  to  be  heard,  that  the  only  way  not  to  be 
heard  was  not  to  speak  at  all." 


162 


ACOUSTICS. 


4.  Reflection  of  Sound. — When  a  sound-wave 
strikes  against  the  surface  of  another  medium,  a 
portion  goes  on  while  the  rest  is  reflected. 


FIG.  122. 


Refraction  of  Sound. 

(1.)  THE  LAW  is  that  of  Motion  ; — the  angle  of 
incidence  is  equal  to  that  of  reflection.*  If  the  re- 
flecting surface  be  very  near,  the  reflected  sound  will 
join  the  direct  one  and  strengthen  it.  This  accounts 
for  the  well-known  fact  that  a  speaker  can  be  heard 

*  Domes  and  curved  walls  reflect  sound  as  mirrors  do  light.  Thus,  in 
the  gallery  under  the  dome  of  St.  Paul's  Cathedral,  London,  persons  stand- 
ing close  to  the  wall  can  whisper  to  each  other  and  be  heard  at  a  great 
distance.— Two  persons,  placed  with  their  backs  to  each  other,  at  the  foci 
of  an  oval  room,  or  "Whispering  G-allery,"  can  carry  on  a  conversation 
that  will  be  inaudible  to  spectators  standing  between  them.— The  covered 
recesses  on  the  opposite  sides  of  a  street,  or  the  arches  of  a  stone  bridge, 
oftentimes  reflect  sound  so  as  to  enable  persons  seated  at  the  foci  to  con- 
verse in  whispers  while  loud  noises  are  being  made  in  the  open  space 
between  these  semi-domes. 


REFLECTION     OF     SOUND. 


163 


more  easily  in  a  room  than  in  the  open  air,  and  that 
a  smooth  wall  back  of  the  stand  re-enforces  the  voice. 
The  old-fashioned  "sounding-boards"  were  by 


no 


FIG.  123. 


Reflection  of  Sound. 

means  inefficient,  however  singular  may  have  been 
their  appearance. 

By  revolving  a  disk  of  card-board  from  which  a 
pair  of  sectors  have  been  cut  out,  and  blowing 
against  it  with  a  trumpet  or  whistle,  a  person  sta- 
tioned at  the  proper  angle  will  notice  a  beating 


164  ACOUSTICS. 

sound  due  to  successive  reflection  and  transmission 
of  the  waves. 

(2.)  ECHOES  are  produced  where  the  reflecting 
surface  is  so  distant  that  we  can  distinguish  the 
reflected  from  the  direct  sound.  If  the  sound  be 
short  and  quick,  this  requires  at  least  fifty  or  sixty 
feet ;  but  if  it  be  an  articulate  one,  as  in  ordinary 
speech,  more  than  a  hundred  feet  are  necessary.  It 
is  possible  to  pronounce  and  hear  distinctly  about 
five  syllables  in  a  second;  1,120  ft.  (the  velocity  at 
a  medium  temperature)  -=-  5  =  224  ft.*  If  the  wave 
travel  224  feet  in  going  and  returning,  the  ad- 
vancing and  returning  sounds  will  not  blend,  and 

*  When  several  parallel  surfaces  are  properly  situated,  the  echo  may  be 
repeated  backward  and  forward  in  a  surprising  manner.  In  Princeton, 
Ind.,  there  is  an  echo  between  two  buildings  that  will  return  the  word 
"Knickerbocker"  twenty  times.  So  many  persons  visited  the  place  tlu.t 
the  city  council  forbade  the  use  of  the  echo  after  9  o'clock  at  night.— At 
Woodstock,  England,  an  echo  returns  seventeen  syllables  by  day  and 
twenty  by  night.  The  reflecting  surface  is  distant  about  2,300  feet,  and  a 
sharp  ha!  will  come  back  a  ringing  ha,  ha,  ha!  —The  echo  is  often  softened, 
as  in  the  Alpine  regions,  where  it  warbles  a  beautiful  accompaniment  to 
the  shepherd's  horn.— The  celebrated  echo  of  the  Metelli  at  "Rome  is  said 
to  have  been  capable  of  distinctly  repeating  the  first  line  of  the  ^Eneid 
eight  times.— In  Fairfax  County,  Va.,  is  an  echo  which  will  return  twenty 
notes  played  on  a  flute.— The  tick  of  a  watch  may  be  heard  from  one  end 
of  the  Church  of  St.  Albans  to  the  other.— At  Carisbrook  Castle,  Isle  of 
Wight,  is  a  well  210  feet  deep  and  twelve  feet  wide,  lined  with  smooth 
masonry.  When  a  pin  is  dropped  into  the  well  it  is  distinctly  heard  to 
strike  the  water.— In  certain  parts  of  the  Colosseum  at  London  the  tearing  of 
paper  sounds  like  the  patter  of  hail,  while  a  single  exclamation  comes  back 
a  peal  of  laughter.— The  dome  of  the  Baptistery  of  the  Cathedral  at  Pisa 
has  a  wonderful  echo.  During  some  experiments  there,  the  author  found 
every  noise,  even  the  rattle  of  benches  on  the  pavement  below,  to  be  reflected 
back  as  if  from  an  immense  distance  and  to  return  mellowed  and  softened 
into  music.— An  interesting  illustration  of  the  reflection  of  sound  is  found 
at  the  so-called  Echo  River,  of  the  Mammoth  Cave,  Ky.  Sounding  in  succes- 
sion the  notes  G,  E,  C,  at  the  middle  of  the  tunnel,  the  boatman  receives  the 
echoes,  all  mingled  into  a  full  chord,  for  eight  or  ten  seconds  afterward. 


MUSICAL     SOUNDS.  165 

the  ear  will  be  able  to  detect  an  interval  between 
them.  A  person  speaking  in  a  loud  voice  squarely 
in  front  of  a  large  smooth  wall  112  feet  distant, 
can  distinguish  the  echo  of  the  last  syllable  he 
utters;  at  224  feet,  the  ]ast  two  syllables,  etc. 

(3.)  DECREASE  OF  SOUND  BY  REFLECTION. — If  we 
strike  the  bell,  represented  in  Fig.  120,  before  a 
vacuum  is  produced,  we  shall  find  a  marked  differ- 
ence between  its  sound  under  the  glass  receiver  and 
in  the  open  air.  Floors  are  deadened  with  tan-bark 
or  mortar,  since  as  the  sound-wave  passes  from  par- 
ticle to  particle  of  the  unhomogeneous  mass,  it  be- 
comes weakened  by  partial  reflection.  The  air  at  night 
is  more  homogeneous,  and  hence  sounds  are  heard 
farther  and  more  clearly  than  in  the  day-time. 

(4.)  ACOUSTIC  CLOUDS  are  masses  of  moist  air  of 
varying  density,  which  act  upon  sounds  as  common 
clouds  do  upon  light,  wasting  it  by  repeated  reflec- 
tions. They  may  exist  in  the  clearest  weather.  To 
their  presence  is  to  be  attributed  the  variation  often 
noticed  in  the  distance  at  which  well-known  sounds, 
as  the  ringing  of  church  bells,  blowing  of  engine- 
whistles,  etc.,  are  heard  at  different  times.* 

5.  Musical  Sounds.  —  (1.)  THE  DIFFERENCE  BE- 
TWEEN NOISE  AND  MUSIC  is  that  between  irregular  and 

*  The  extinction  of  sound  by  such  agencies  is  often  almost  incredible. 
Thus  two  observers  looking  across  the  valley  of  the  Chickahominy  at  the 
battle  of  Q-aines'  Mill  failed  to  hear  a  sound  of  the  conflict,  though  they 
could  clearly  see  the  lines  of  soldiers,  the  batteries,  and  the  flash  of  the 
guns.— These  phenomena  are  ascribed  by  many  to.  an  elevation  or  a  depres- 
sion of  the  wave-front  so  that  the  sound  passes  .above  the  observer  or  is 
stopped  before  it  reaches  Mm.  See  "  Stewart's  Physics,"  p.  141. 


166  ACOUSTICS. 

regular  vibrations.  Whatever  the  cause  which  sets 
the  air  in  motion,  if  the  vibrations  are  uniform  and 
rapid  enough,  the  sound  is  musical.  If  the  ticks  of 
a  watch  could  be  made  with  sufficient  rapidity,  they 
would  lose  their  individuality  and  blend  into  a  mu- 
sical tone.  If  the  puffs  of  a  locomotive  could  reach 
fifty  or  sixty  a  second,  its  approach  would  be  her- 
alded by  a  tremendous  organ-peal.* 

(2.)  PITCH  depends  on  the  rapidity  of  the  vibra- 
tions. Thus,  if  we  hold  a  card  against  the  cogs  of 
a  rapidly-revolving  wheel,  we  shall  obtain  a  clear 
tone ;  and  the  faster  the  wheel  turns,  the  shriller 
the  tone,  i.  e.,  the  higher  the  pitch. 

(3.)     THE     NUMBER     OF     VIBRATIONS     PER     SECOND     IS 

determined  by  an  instrument  called  the  siren.  It 
consists  of  a  cylindrical  box  (Figs.  124  and  125),  the 
top  of  which  is  pierced  with  a  series  of  holes.  Over 
this  is  a  plate  with  a  corresponding  series,  fixed  to  a 
vertical  rod,  which  is  pivoted  on  the  lower  plate  so 
as  to  revolve  easily.  It  is  provided  with  an  endless 
screw  (Fig.  125),  which  operates  some  clock-work. 

*  The  pavement  of  London  is  largely  composed  of  granite  blocks,  four 
inches  in  width.  A  cab- wheel  jolting  over  this  at  the  rate  of  eight  miles 
per  hour  produces  a  succession  of  35  sounds  per  second.  These  link  them- 
selves into  a  soft,  deep  musical  tone,  that  will  bear  comparison  with  notes 
derived  from  more  sentimental  sources,  even  though  it  may  seem  con- 
fused to  a  hearer  in  its  midst.  This  tendency  of  Nature  to  music  is 
something  wonderful.  "Even  friction,"  says  Tyndall,  "is  rhythmic."  A 
bullet  flying  through  the  air  sings  softly  as  a  bird.  The  limbs  and  leaves 
of  trees  murmur  as  they  sway  in  the  breeze.  Palling  water,  singing  birds, 
sighing  winds,  every- where  attest  that  the  same  Divine  love  of  the  beautiful 
which  causes  the  rivers  to  wind  through  the  landscape,  the  trees  to  bend  in 
a  graceful  curve— the  line  of  beauty— and  the  rarest  flowers  to  bud  and 
blossom  where  no  eye  save  His  may  see  them,  delights  also  in  the  anthem 
of  praise  which  Nature  sings  for  His  ear  alone. 


MUSICAL    SOUNDS. 


167 


On  the  dial  (Fig.  124),  we  can  see  the  number  of 
turns  made  by  the  upper  disk.  The  holes  in  the  two 
disks  are  oppositely  inclined,  so  that  when  a  current 
of  air  is  forced  in  from  below  it  passes  up  through 
the  openings  in  the  lower  disk,  and  striking  against 


FIG.  124. 


FIG.  125 


The  Siren. 

the  sides  of  those  in  the  upper  disk,  causes  it  to  re- 
volve. As  that  turns,  it  alternately  opens  and  closes 
the  orifices  in  the  lower  disk,  and  thus  converts  the 
steady  stream  of  air  into  uniform  puffs.  At  first 
they  succeed  each  other  so  slowly  that  they  may  be 
counted.  But,  as  the  motion  increases,  they  link 
themselves  together,  and  pass  into  a  full,  melodious 
note.  As  the  velocity  augments,  the  pitch  rises, 


168  ACOUSTICS. 

until  the  music  becomes  painfully  shrill.  Diminish 
the  speed,  and  the  pitch  falls. 

To  find,  therefore,  the  number  of  vibrations  in  a 
given  sound,  force  the  air  through  the  siren  until  the 
required  pitch  is  reached.  See  on  the  dial,  at  the  end 
of  a  minute,  the  number  of  revolutions  of  the  disk. 
Suppose  the  number  of  holes  in  a  disk  to  be  10,  and 
the  tone  produced  to  be  in  unison  with  that  of  a  (73 
tuning-fork.  The  number  of  revolutions  indicated  on 
the  dial  at  the  end  of  a  minute  is  found  to  be  1,536. 
There  were  10  puffs,  or  10  waves  of  sound,  for  each 
revolution.  1,536  x  10  =  15,360.  Dividing  this  by 
60,  we  have  256,  the  number  per  second.  Increasing 
now  the  blast  until  the  tone  produced  is  in  unison 
with  a  (74  tuning-fork,  the  octave  above  the  first,  the 
number  of  vibrations  per  second  is  found  to  be  512. 
Hence  the  octave  of  a  tone  is  caused  by  double  the 
number  of  vibrations. 

(4.)  To  FIND  THE  LENGTH  OF  THE  WAVE. — Suppose 
the  air  in  the  last  experiment  was  of  such  a  temper- 
ature that  the  foremost  sound-wave  traveled  1,120 
feet  in  a  second.  In  that  space  there  were  256 
sound-waves.  Dividing  1,120  by  256,  we  have  4|  ft. 
as  the  length  of  each.  We  thus  find  the  wave-length 
by  dividing  the  velocity  by  the  number  of  vibrations 
per  second.  As  the  pitch  is  elevated  by  rapidity  of 
vibration,  we  perceive  that  the  low  tones  in  music 
are  produced  by  the  long  waves  and  the  high  tones 
by  the  short  ones.* 

*  The  aerial  waves  are  seemingly  shortened  when  the  source  of  sound 
is  approaching,  whether  by  its  own  motion  or  the  hearer's,  and  lengthened 


INTERFEKENCE     OF     SOUND-WAVES.  169 

(5.)  TONES  IN  UNISON. — If  the  string  of  a  violin, 
the  cord  of  a  guitar,  the  parchment  of  a  drum,  and 
the  pipe  of  an  organ,  produce  the  same  tone,  it  is  be- 
cause they  are  executing  the  same  number  of  vibra- 
tions per  second.  If  a  voice  and  a  piano  perform  the 
same  music,  the  steel  strings  of  the  piano  and  the 
vocal  cords  of  the  singer  vibrate  together  and  send 
out  sound-waves  of  the  same  length. 

6.  Interference  of  Sound-waves.  —  Just  as  two 
water-waves  by  meeting  in  opposite  phases  may  de- 
stroy one  another,  so  by  a  proper  adjustment  two 
sound-waves  may  be  made  to  interfere,  and,  if  ex- 
actly equal  and  opposite,  to  produce  silence.  Fig. 
126  represents  a  piece  of  apparatus  intended  to  show 
this.  Let  a  tone,  such  as  <73,  be  sounded  in  the 
mouthpiece  at  a.  The  waves  divide  at  the  end  of 
the  first  India-rubber  tube  and  reunite  on  entering  the 
second,  before  entering  the  ear  at  5.  One  branch  o£ 
the  channel  is  made  of  two  tubes,  One  of  which  slides 
over  the  other  so  that  the  branch  may  be  lengthened 
at  will.  If  it  be  pulled  up  so  high  that  the  waves 
passing  through  it  shall  traverse  a  half  wave-length 
more  in  distance  than  those  in  th6  fixed  branch,  oppo- 
site phases  will  meet  where  they  reunite,  and  the  list- 


is  receding.  In  the  former  case,  the  tone  of  the  sound  is  more  acute ; 
in  the  latter,  graver.  This  is  strikingly  illustrated  when  a  swift  train  rushes 
past  a  station,  the  whistle  blowing.  While  the  cars  are  approaching,  a  per- 
son hears  a  note  somewhat  sharper ;  after  it  has  passed,  one  somewhat  flatter 
than  the  true  note.  Still  more  obvious  is  the  change  when  two  trains  pass 
each  other.  A  person  unfamiliar  with  the  arrangement  would  suppose  a 
different  bell  was  rung.  In  one  case  more  and  in  the  other  fewer  waves 
reach  the  ears  in  a  second, 


170 


ACOUSTICS. 


ener  notices  great  weakening  of  the  sound.  By 
proper  handling,  the  sound  received  may  be  made  to 
become  alternately  strong  and  weak  without  any 
change  in  the  sound  given.* 


FIG.  126. 


Tube  for  Interference  of  Sound. 


If  we  strike  a  tuning-fork  and  turn  it  slowly 
around  before  the  ear,  we  shall  find  four  points 
where  the  interference  of  the  sound-waves  causes 

*  We  can  not  produce  complete  extinction  because,  1.  The  sound  is  con- 
ducted not  only  by  the  inclosed  air,  but  by  the  solid  tube  also ;  2.  There 
is  loss  by  friction  in  the  longer  branch ;  3.  There  is  loss  by  leakage  between 
the  tubes  that  slide  against  each  other. 


VIBRATIONS     OF     COEDS.  171 

great  weakening.  The  two  prongs  swing  alternately 
toward  and  from  each  other.  When  a  condensation 
is  produced  between  the  prongs,  a  rarefaction  is  pro- 
duced on  their  outer  sides.  Certain  lines  can  be 
found  where  these  interfere. 

If  two  forks  are  nearly  but  not  quite  in  unison, 
the  waves  from  them  are  unequal  in  length.  They 
alternately  conjoin  and  oppose  each  other,  producing 
"beats."  These  are  often  noticed  in  the  sound  from 
a  large  bell,  the  opposite  sides  of  which  are  not  quite 
equally  elastic.  A  pair  of  mistuned  organ-pipes  pro- 
duces a  similar  effect,  and  the  discord  of  an  inferior 
piano,  or  indeed  all  discord,  is  due  to  beats. 

7.  Vibrations  of  Cords. — Let  ab  be  a  stretched 
cord  made  to  vibrate.  The  motion  from  e  to  d  and 

PIG.  127. 

d 


Vibrating  Cord. 

back  again  is  termed  a  vibration;  that  from  e  to  d, 
a  half-vibration.  The  distance,  cd,  from  the  middle 
to  either  of  the  extreme  positions  is  the  amplitude. 
(1.)  THE  SONOMETER  is  an  instrument  used  to  in 
vestigate  the  laws  of  vibration  of  stretched  cords. 
It  consists  of  two  cords  stretched  by  weights,  P, 
across  fixed  bridges,  A  and  B.  The  movable  bridge, 
Z>,  serves  to  lengthen  or  shorten  the  vibrating  part 
of  either  cord.  Beneath  is  a  resonance  box,  to  which 
the  vibrations  are  conducted  by  the  bridges.  This  is 
the  body  whose  sound  is  chiefly  heard. 


172  ACOUSTICS. 

(2.)  THREE  LAWS. — I.  The  number  of  vibrations 
per  second  increases  as  the  length  of  the  cord  de- 
creases. By  plucking  the  cord  with  the  finger,  or 
drawing  ^a  violin  bow  across  it,  make  it  vibrate,  giving 
the  note  of  the  entire  string.  Place  the  bridge  D  at 
the  center  of  the  cord,  and  the  sound  will  be  the  oc- 
tave above  the  former.  Thus,  by  taking  one  half  the 
length  of  the  cord  we  double  the  number  of  vibra- 
tions.— Examples:  If  an  entire  cord  make  20  vibra- 
tions per  second,  one  half  will  make  40,  and  one 

FIG.  128. 


third,  60. — The  violin  or  guitar  player  elevates  the 
pitch  of  a  string  by  moving  his  finger,  thus  shorten- 
ing the  vibrating  portion. — In  the  piano,  harp,  etc., 
the  long  and  the  short  strings  produce  the  low  and 
the  high  notes  respectively. 

II.  The  number  of  vibrations  per  second  increases 
as  the  square  root  of  the  tension.  The  cord  when 
stretched  by  1  Ib.  gives  a  certain  tone.  To  double 
the  number  of  vibrations  and  obtain  the  octave  re- 
quires 4  Ibs.  Stringed  instruments  are  provided  with 


VIBRATIONS     OF     CORDS.  173 

keys,  by  which  the  tension  of  the  cord  and  the  cor- 
responding pitch  may  be  increased  or  diminished. 

III.  The  number  of  vibrations  per  second  decreases 
as  the  square  root  of  the  weight  of  the  cord  increases. 
If  two  strings  of  the  same  material  be  equally 
stretched,  and  one  have  four  times  the  weight  of 
the  other,  it  will  vibrate  only  half  as  often.  In  the 
violin  the  bass  notes  are  produced  by  the  thick 
strings.  In  the  piano  fine  wire  is  coiled  around  the 
heavy  strings  to  increase  their  weight. 


FIQ.  129. 


Production  of  Two  Segments. 

(3.)  NODES. — In  the  experiments  just  described,  the 
cord  is  shortened  by  means  of  a  firm,  movable  bridge. 
If,  instead,  we  rest  a  feather  lightly  on  the  string, 
and  draw  the  bow  over  one  half,  the  cord  will 
vibrate  in  two  portions  and  give  the  octave  as  be- 
fore. Remove  the  feather,  and  it  will  continue  to 
vibrate  in  two  parts  and  to  yield  the  same  tone. 
We  can  show  that  the  second  half  vibrates  by  plac- 
ing across  that  portion  a  little  paper  rider.  On  draw- 
ing the  bow  it  will  be  thrown  off.  Hold  the  feather 


174 


ACOUSTICS. 


so  as  to  separate  one  third  of  the  string  and  cause 
it  to  vibrate  ;  the  remainder  of  the  cord  will  vibrate 
in  two  segments.  When  the  feather  is  removed,  the 


FIG.  131. 


Production  of  Three  Segments. 

entire  cord  will  vibrate  in  three  different  parts  of 
equal  length,  separated  by  stationary  points  called 
nodes.  This  may  be  shown  by  the  riders ;  the  one 
at  the  node  remains,  while  the  others  are  thrown  off. 
(4.)  ACOUSTIC  FIGURES.  —  Sprinkle  fine  sand  on  a 
metal  plate.  Place 
the  finger-nail  on 
one  edge  to  stop 
the  vibration  at 
that  point,  as  the 
feather  did  in  the 
last  experiment, 
and  draw  the  bow 
lightly  across  the 

opposite  edge.  The  sand  will  be  tossed 
away  from  the  vibrating  parts  of  the 
plate  and  will  collect  along  the  nodal  vibration  of  a  Plate. 


VIBRATIONS     OF     COKDS. 


175 


FIG.  132. 


lines,  which  divide  the  large  square.  It  is  wonder- 
ful to  see  how  the  sand  will  seemingly  start  into  life 
and  dance  into  line  at  the  touch  of  the  bow.  Fig. 
132  shows  some  of 
the  beautiful  patterns 
obtained  by  Chladni. 
(5.)  HAEMONICS.* — 
Whenever  a  cord  vi- 
brates, it  separates 
into  segments  at  the 
same  time.  Thus  we 
have  the  full  or  fun- 
damental note  of  the 
entire  string,  and  su- 
perposed upon  it  the 
higher  notes  pro- 
duced by  the  vibrat- 
ing parts.  These  are  called  overtones  or  harmonics. 
The  mingling  of  the  two  classes  of  vibrations  deter- 
mines the  quality  of  the  sound,  and  enables  us  to 
distinguish  the  music  of  different  instruments. 

(6.)  NODES  OF  A  BELL. — Let  the  heavy  circle  in 
Fig.  133  represent  the  circumference  of  a  bell  when 
at  rest.  Let  the  hammer  strike  at  a,  Z>,  c,  or  d.  At 


Chladni's  Figures. 


*  Press  gently  but  firmly  down  the  notes  C,  Q-,  and  C,  in  the  octave 
above  middle  C,  on  the  piano-forte.  Without  releasing  these  keys,  give  to 
C  below  middle  C  a  quick,  hard  blow.  The  damper  will  fall,  and  the  sound 
will  stop  abruptly.  At  the  same  instant  a  low,  soft  chord  will  be  heard. 
This  comes  from  the  three  strings  whose  dampers  are  raised,  leaving  them 
free  to  sound  in  sympathy  with  the  overtones  of  the  lower  C,  which  sounds 
are  identical  with  their  own.— When  a  goblet  or  wine-glass  is  tapped  with 
a  knife-blade,  we  can  distinguish  three  sounds,  the  fundamental  and  two 
harmonics. 


176  ACOUSTICS. 

one  moment,  as  the  bell  vibrates,  it  forms  an  oval 
with  ab,  at  the  next  with  cd,  for  its  longest  diam- 
eter. When  it  strikes  its  deepest  note,  the  bell  vi- 
brates in  four  segments,  with 
n,  n,  n,  n}  as  the  nodal  points, 
whence  nodal  lines  run  up 
from  the  edge  to  the  crown 
of  the  bell.  It  tends,  however, 
to  divide  into  a  greater  num- 
ber of  segments,  especially  if 
it  is  very  thin,  and  to  produce 
harmonics.  The  overtones 

'    Vibration  of  a  Bell.  wMch      accompany     the      deep 

tones  of  the  bell  are  frequently  very  striking,  even 
in  a  common  call-bell,  and  often  make  it  hard  to  de- 
termine at  once  what  is  its  fundamental.  Usually 
they  die  away  sooner  than  the  fundamental. 

(7.)  NODES  OF  A  SOUNDING-BOARD. — The  case  of  a 
violin  or  guitar  is  composed  of  thin  wooden  plates 
which  divide  into  vibrating  segments,  separated  by 
nodal  lines  according  to  the  pitch  of  the  note 
played.  The  inclosed  air  vibrating  in  unison  with 
these,  re-enforces  the  sound  and  gives  it  fullness  and 
richness. 

(8.)  MUSICAL  SCALE. — The  lowest  tone  that  can  be 
distinctly  perceived  as  musical  by  most  ears  is  pro- 
duced by  32  vibrations  per  second.  This  is  called 
C0.  The  octave  above  this  is  C^  64  vibrations;  the 
double  octave,  C2,  128  vibrations,  etc.  If  a  string  be 
stretched  so  as  to  give  C8,  the  tones  of  the  common 
musical  scale  between  this  and  C3  are  obtained  from 


VIBRATION     OF     COLUMNS     OF     AIR.  177 

the   parts   of  the  string  indicated  by  the  following 
fractions : 

C2         Da         Ea         F8         G2         A2         B3         03 
1          "***!*** 

As  the  number  of  vibrations  varies  inversely  as 
the  length  of  the  cord,  we  have  only  to  invert  these 
fractions  to  obtain  the  relative  number  of  vibrations 
per  second;  thus,* 

C2         D3         E8         F8         G3         A2         B2         C8 

1  t  I  t  I  I  ¥          2 

128        144       160        170       192        214        240       256 

8.  Vibration  of  Columns  of  Air. — If  a  tuning-fork 
be  excited  and  its  prongs  be  held  before  the  open 
end  of  a  tube  of  proper  length,  the  sound  will  be- 
come much  louder.     If  the  pitch  of  the  fork  is  C4, 
512  vibrations,  the  length  of  such  a  tube,  open  at 
both  ends,  is  about  18  inches;    if  open  at  only  one 
end,  6|-  inches.    A  hollow  globe  of  proper  size,  with 
an  opening  on  one  side,  will  respond  in  like  manner. 
Such  bodies  are  called  resonators. 

9.  Wind   Instruments  produce  sounds  by  the  vi- 
bration  of  the   columns  of    air  which   they  inclose. 
An  organ-pipe  is  merely  a  tube-resonator.     The  sound- 


*  In  this  table,  "C3  =  256  vibrations"  represents  the  middle  C  of  a 
piano-forte.  This  number  is  purely  arbitrary.  The  so-called  "  concert- 
pitch  ''  varies  in  different  countries.  The  Stuttgart  Congress  of  1834  fixed 
the  standard  tuning-fork  —  middle  A— at  440  vibrations  per  second;  while 
the  Paris  Conservatory  (1859)  gave  to  middle  A  437.5.  The  pitch  agreed  upon 
as  an  international  standard  by  the  Vienna  Conference  in  1885  for  A=  435 
vibrations.  This  was  adopted  by  the  Piano  Manufacturers'  Association  of 
New  York  and  vicinity  in  1891.  The  ratio  of  the  different  numbers  is  identi- 
cal, whatever  the  pitch. 


178 


ACOUSTICS. 


waves  in  organ-pipes  are  set  in  motion  by  either 
fixed  mouth-pieces  or  vibrating  reeds.  The  air  is 
forced  from  the  bellows  into  the  tube  P,  through  the 
vent  i,  and  striking  against  the  thin  edge  a,  produces 
a  flutter.  The  column  of  air  above,  thrown  into  vi- 

Fio.  134. 


Organ-pipes. 

bration,  re-enforces  the  sound  and  gives  a  full  mu- 
sical tone.  The  length  of  the  pipe,  if  open,  should  be 
•J  wave  length  corresponding  to  the  pitch  to  which 
it  responds;  if  closed,  £  wave  length.  If  a  tuning- 
fork  which  produces  this  pitch  be  held  at  &  while  vi- 
brating, the  sound  will  at  once  become  much  stronger. 


CO-VIBKATION.  179 

The  air  co-vibrates,  whatever  may  be  the  source  of 
sound,  if  only  the  pitch  be  properly  adjusted. 

1C.  Co-vibration. — We  have  already  seen  (p.  158) 
how  one  tuning-fork  may  co-vibrate  with  another 
through  the  medium  of  the  air.  Vibrations  thus 
produced  are  often  called  sympathetic,  and  bodies 
which  thus  strengthen  sound  are  said  to  be  resonant. 
Produce  a  musical  tone  with  the  voice  near  a  piano, 
and  a  certain  wire  will  seem  to  select  that  sound  and 
respond  to  it.  Change  the  pitch,  and  the  first  string 
will  cease,  while  another  replies.  If  a  hundred  tun- 
ing-forks of  different  tones  are  sounding  at  the  foot 
of  an  organ-pipe,  'it  will  strengthen  the  sound  of  the 
one  to  which  it  can  reply,  and  answer  that  alone. 
Helmholtz  has  applied  this  principle  to  the  construc- 
tion of  the  resonance  globe,  an  instrument  which 
will  respond  to  a  particular  harmonic  in  a  compound 
tone,  and  strengthen  it  so  as  to  make  it  audible. 

(1.)  SENSITIVE  FLAMES.  —  Flames  are  sensitive  to 
sound.  At  an  instrumental  concert  the  gas-lights 
vibrate  with  certain  pulsations  of  the  music.  This 
is  noticeable  when  the  pressure  of  gas  is  so  great 
that  the  flame  is  just  on  the  verge  of  flaring,  and 
the  vibration  of  the  sound-wave  is  sufficient  to  "  push 
it  over  the  precipice."  * 

(2.)  SINGING  FLAMES. — If  we  lower  a  glass  tube 
over  a  small  gas-jet,  we  soon  reach  a  point  where 

*  Prof.  Barrett,  of  Dublin,  describes  a  peculiar  jet  -which  is  so  sensitive 
that  it  trembles  and  cowers  at  a  hiss,  like  a  human  being,  beats  time  to 
the  ticking  of  a  watch,  and  is  violently  agitated  by  the  rumpling  of  a  silk 


180 


ACOUSTICS. 


FIG.  135. 


the  flame  leaps  spontaneously  into  song.  At  first 
the  sound  seems  remote,  but  gradually  approaches 
until  it  bursts  into  an  almost  full  song.  The  length 
of  the  tube  and  the  size  of  the  jet  determine  the 
pitch  of  the  note.*  The  flame,  owing  to  the  friction 

at  the  mouth  of  the 
pipe,  is  thrown  into 
vibration.  The  air 
vibrates  in  unison 
with  the  jet,  and, 
like  that  in  the  or- 
gan-pipe, selects  the 
tone  corresponding 
to  the  length  of  the 
tube. 

11.  The  Phono- 
graph is  an  instru- 
ment for  recording 
and  reproducing 
the  vibrations  of 
sound.  Its  essen- 
tial features  are  as 
follows : 

(1.)  A  metallic 
cylinder  which  can 
be  rotated  on  a 


Singing  Flame. 


screw  as  axis,  so  as  to  secure  motion  that  is  side- 
ward as  well  as  rotary. 

(2.)  A  hollow  cylinder  of  wax  which  fits  over  the 

*  The  jets  are  easily  made  by  drawing  out  glass  tubing  to  a  fine  point 
over  a  spirit-lamp. 


THE     PHONOGRAPH. 


181 


metallic  cylinder,  and  may  be  removed  after  receiv- 
ing impressions  from  a  source  of  sound. 

(3.)  A  mouth-piece  into  which  the  speaker  vo- 
calizes. At  the  bottom  of  this  is  an  elastic  disk, 
which  is  set  into  vibration  by  the  voice. 

(4.)  A  lever  which  is  actuated  by  the  disk.  At 
one  end  of  it  is  a  specially  prepared  needle,  which 
makes  indentations  upon  the  rotating  cylinder  of 


wax. 


Pia.  136. 


The  Phonograph 

After  the  line  of  indentations  has  been  made  on 
the  wax,  the  cylinder  is  brought  back  to  its  first 
position.  On  turning  it,  the  needle,  pressing  on  the 
serrated  surface,  receives  vibratory  motion  like  that 
which  had  been  given  it  by  the  voice.  This  is  re- 
ceived by  the  disk,  and  the  instrument  thus  talks 
out  what  had  been  talked  into  it. 


182  ACOUSTICS. 

There  are  usually  two  mouth-pieces,  interchange- 
able in  position,  one  of  which  is  used  in  speaking  to 
the  phonograph,  and  the  other  in  giving  out  what 
this  has  to  say.  Each  is  specially  adapted  to  the 
work  it  has  to  do.  The  metallic  cylinder  is  rotated 
by  means  of  an  electric  motor.  The  arm  which  car- 
ries the  mouth-pieces  is  provided  with  a  turning 
tool  for  smoothing  the  wax  before  this  receives  the 
record  from  the  voice. 

In  Fig.  136,  the  phonograph  is  shown  ready  to 
talk.  A  conical  speaking-trumpet  is  fixed  upon  the 
mouth-piece,  so  that  the  sound  may  be  strengthened 
by  co-vibration.  The  wax  cylinder  can  be  kept  any 
length  of  time,  and  be  made  to  speak  out  its  message 
repeatedly.  The  phonograph  reproduces  so  accurately 
the  sounds  it  has  received,  that  even  the  peculiarities 
which  result  from  the  special  quality  of  the  speaker's 
Voice  can  be  recognized. 

12.  The  Human  Ear  is  also  an  instrument  for 
receiving  sound  vibrations,  which  affect  the  auditory 
nerve  and  produce  sensation  thus  at  the  base  of  the 
brain.  ("Hygienic  Physiology,"  p.  216.) 

(1.)  RANGE  OF  THE  EAR. — JSTo  definite  limit  can 
be  assigned  to  the  range  through  which  musical 
sounds  are  perceptible.  The  highest  limit  has  been 
roughly  estimated  to  be  about  38,000,  and  the  low- 
est, 16,  vibrations  per  second.  When  the  number  of 
impressions  on  the  ear  in  each  second  is  less  than 
15  or  16,  we  become  able  to  perceive  them  sepa- 
rately. To  be  musical  they  must  come  fast  enough 
to  appear  to  coalesce.  From  16  to  33,000  is  about 


THE     EAR.  183 

eleven  octaves.  The  capacity  to  hear  the  higher 
tones  varies  in  different  persons.  A  sound  audible 
to  one  may  be  silence  to  another.  Some  ears  can  not 
distinguish  the  squeak  of  a  bat  or  the  chirp  of  a 
cricket,  while  others  are  acutely  sensitive  to  these 
shrill  sounds.  Indeed,  the  auditory  nerve  seems  gen- 
erally more  alive  to  the  short,  quick  vibrations  than 
to  the  long,  slow  ones.  The  whirr  of  a  locust  is 
much  more  noticeable  than  the  sighing  of  the  wind 
through  the  trees.* 

(2.)  THE  ABILITY  OF  THE  EAR  TO  DETECT  AND  AN- 
ALYZE SOUND  is  wonderful  beyond  comprehension. 
Sound-waves  chase  one  another  up  and  down  through 
the  air,  superposed  in  entangled  pulsations,  yet  a  cyl- 
inder not  larger  than  a  quill  conveys  them  to  the 
ear,  and  each  string  of  that  wonderful  harp  selects 
its  appropriate  sound,  and  repeats  the  music  to  the 
soul.  Though  a  thousand  instruments  be  played  at 
once,  there  is  no  confusion,  but  each  is  heard,  and 
all  blend  in  harmony,  f 

*  To  this,  however,  there  are  remarkable  exceptions.  The  author  knows 
a  lady  who  is  insensible  to  the  higher  tones  of  the  voice,  but  acutely  sensi- 
tive to  the  lower  ones.  Thus,  on  one  occasion,  being  in  a  distant  room,  she 
did  not  notice  the  ringing  of  the  bell  announcing  dinner,  but  heard  the 
noise  the  bell  made  when  returned  to  its  place  on  the  shelf.  v 

t  "Is  not  the  ear  the  most  perfect  sense?  A  needle-woman  will  distin- 
guish by  the  sound  whether  it  is  silk  or  cotton  that  is  torn.  Blind  people 
recognize  the  age  of  persons  by  their  voices.  An  architect,  comparing  the 
length  of  two  lines  separated  from  each  other,  if  he  estimate  within  ^,  we 
deem  very  accurate ;  but  a  musician  would  not  be  considered  very  precise 
who  estimated  within  a  quarter  of  a  note  (128  -4-  30  =  4  nearly).  In  a  large 
orchestra,  the  leader  will  distinguish  each  note  of  each  instrument.  We 
recognize  an  old-time  friend  by  the  sound  of  his  voice,  when  the  other 
senses  utterly  fail  to  recall  him.  The  musician  carries  in  his  ear  the  idea 
of  the  musical  key  and  every  tone  in  the  scale,  though  he  is  constantly 
hearing  a  multitude  of  sounds," 


184  ACOUSTICS. 


'   PRACTICAL     QUESTIONS. 

1.  "Why  can  not  the  rear  of  a  long  column  of  soldiers  keep  time  to  the 
music  in  front? 

2.  Three  minutes  elapse  between  the  flash  and  the  report  of  a  thunder- 
bolt; how  far  distant  is  it? 

3.  Five  seconds  expire  between  the  flash  and  the  report  of  a  gun ;  what 
is  its  distance? 

4.  Suppose  a  speaking-tube  should  connect  two  villages  ten  miles  apart ; 
how  long  would  it  take  the  sound  to  travel  ? 

5.  The  report  of  a  pistol-shot  was  returned  to  the  ear  from  the  face  of 
a  cliff  in  four  seconds ;  what  was  the  distance  ? 

6.  What  is  the  cause  of  the  difference  between  the  voice  of  man  and 
woman?    A  base  and  a  tenor  voice? 

7.  What  is  the  number  of  vibrations  per  second  necessary  to  produce 
the  fifth  tone  of  the  scale  of  C3  ? 

8.  What  is  the  length  of  each  sound-wave  in  that  tone  when  the  tem- 
perature is  at  zero? 

9.  What  is  the  number  of  vibrations  in  the  fourth  tone  above  Ca  ? 

30.  If  a  meteor  were  to  explode  at  a  height  of  60  miles,  would  it  be 
possible  for  its  sound  to  be  heard  at  sea-level? 

11.  A  stone  is  let  fall  into  a  well,  and  in  four  seconds  is  heard  to  strike 
the  bottom;  how  deep  is  the  well? 

12.  What  time  would  be  required  for  a  sound  to  travel  five  miles  in 
the  still  water  of  a  lake? 

13.  Does  sound  travel  faster  at  the  foot  than  at  the  top  of  a  mountain  ? 

14.  Why  is  an  echo  weaker  than  the  original  sound? 

15.  Why  is  it  so  fatiguing  to  talk  through  a  speaking-trumpet? 

16.  Why  will  the  report  of  a  cannon  fired  in  a  valley  be  heard  on  the 
top  of  a  neighboring  mountain,  better  than  one  fired  on  the  top  of  a  mount- 
ain will  be  heard  in  the  valley? 

17.  Why  do  our  footsteps  in  unfurnished  dwellings  sound  so  startlingly 
distinct  ? 

18.  Why  do  the  echoes  of  an  empty  church  disappear  when  the  audi- 
ence assemble? 

19.  What  is  the  object  of  the  sounding-board  of  a  piano? 

20.  During   some   experiments,   Tyndall   found    that   a   certain   sound 
would  pass  through  twelve  folds  of  a  dry  silk  handkerchief,  but  would  be 
stopped  by  a  single  fold  of  a  wet  one.    Explain. 

21.  What  is  the  cause  of  the  musical  murmur  often  heard  near  tele- 
graph lines? 

22.  Why  will  a  variation  in  the  quantity  of  water  in  a  goblet,  when 
this  is  made  to  sound,  cause  a  difference  in   the  tone   produced  by  its 
vibration  ? 

23.  At  what  rate  (in  meters)  will  sound  move  through  air  at  sea-level, 
the  temperature  being  20s  C.? 


SUMMARY.  185 


SUMMARY. 

SOUND  is  produced  by  vibrations.  These  are  transmitted  in 
waves  through  the  air  (60°  F.)  at  sea-level  at  the  rate  of  1,120 
ft.  per  second ;  through  water  four  times,  and  through  iron  fif- 
teen times  as  fast.  In  general,  the  velocity  depends  on  the  re- 
lation between  the  density  and  the  elasticity  of  the  medium ; 
and  the  intensity  is  proportional  to  the  square  of  the  amplitude 
of  the  vibrations.  Sound,  like  light,  may  be  reflected  and  re- 
fracted to  a  focus.  Echoes*  are  produced  by  the  reflection  of 
sound  from  smooth  surfaces,  not  less  than  112  ft.  (about  33 
meters)  distant.  Rapidly-repeated  vibrations  make  a  continuous 
sound ;  regular  and  rapid  vibrations  produce  music ;  irregular 
ones  cause  a  noise. 

The  pitch  of  a  sound  depends  on  the  rapidity  of  the  vibra- 
tions. The  number  of  waves,  and  their  consequent  length  in  a 
given  sound,  is  found  by  means  of  the  siren.  Unison  is  pro- 
duced by  identical  wave-motions.  Any  number  of  sound-waves 
may  traverse  the  air,  as  any  number  of  water-waves  may  the 
surface  of  the  sea,  without  losing  their  individuality.  The  mo- 
tion of  each  molecule  of  air  is  the  algebraic  sum  of  the  several 
motions  it  receives.  Two  systems  of  waves  may  therefore  de- 
stroy or  strengthen  each  other,  according  as  they  meet  in  oppo- 

*  Several  acoustic  phenomena  have  become  of  historical  interest.  (1.) 
Near  Syracuse,  Sicily,  is  a  cave  known  as  the  Ear  of  Dionysius.  A  whisper 
at  the  farther  end  of  the  cavern  is  easily  heard  by  a  person  at  the  entrance, 
though  the  distance  is  200  ft.  Tradition  says  that  the  Tyrant  of  Syracuse 
used  this  as  a  dungeon,  and  was  thus  enabled  to  listen  to  the  conversation 
of  his  unfortunate  prisoners.  (2.)  On  the  banks  of  the  Nile,  near  Thebes, 
is  a  statue  47  ft.  high,  and  extending  7  ft.  below  the  ground.  It  is  called 
the  Vocal  Memnon.  Ancient  writers  tell  us  that  about  sunrise  each  morn- 
ing, there  issued  from  this  gigantic  monolith  a  musical  sound  resembling 
the  breaking  of  a  harp-string.  It  is  now  believed  that  this  was  produced 
by  friction  due  to  unequal  expansion  of  different  parts  under  the  morning 
sun.  (3.)  Near  Mount  Sinai,  in  Arabia,  remarkable  sounds  are  produced  by 
the  sand  falling  down  a  declivity.  The  sand,  which  is  very  white,  fine, 
and  dry,  lies  at  such  an  angle  as  to  be  easily  set  in  motion  by  any  cause, 
such  as  scraping  away  a  little  at  the  foot  of  the  slope.  The  sand  then  rolls 
down  with  a  sluggish  motion,  causing  at  first  a  low  moan,  that  gradually 
swells  to  a  roar  like  thunder,  and  finally  dies  away  as  the  motion 


186  ACOUSTICS. 

site  or  in  similar  phases.  Interference  is  the  mutual  weakening 
of  two  systems  of  waves  which  meet  in  opposite  phases.  Beats 
are  the  effect  produced  by  two  musical  sounds  of  nearly  the 
same  pitch,  which  alternately  interfere  and  coalesce.  The  vibra- 
tions of  a  cord  produce  a  musical  sound,  which  is  re-enforced  by 
a  sounding-board.  The  rate  of  vibration  and  consequent  pitch 
depends  on  the  length,  the  tension,  and  the  weight  of  the  cord. 
A  sounding  body  vibrates  not  only  as  a  whole,  but  also  in  seg- 
ments. Its  vibration  as  a  whole  produces  the  fundamental  tone, 
and  the  additional  vibration  in  segments  gives  rise  to  the  over- 
tones. These  together  form  either  a  complete  or  an  interrupted 
harmonic  series.  The  quality  of  the  compound  sound  depends 
on  the  number,  orders,  relative  intensities,  and  mode  of  com- 
bination of  the  overtones  into  which  it  can  be  resolved.  The 
various  notes  in  the  musical  scale  are  determined  by  fixed  por- 
tions of  the  length  of  the  cord.  The  music  of  a  wind  instru- 
ment is  produced  by  vibrating  columns  of  air.  Resonance  is  a 
sympathetic  vibration  caused  by  one  sonorous  body  acting  on 
another,  through  a  conducting  medium,  as  seen  in  the  resonance 
globe,  etc.  The  voice  is  a  reed  instrument,  with  its  vibrating 
cords  and  resonant  cavity.  The  ear  collects  the  sound-waves. 
It  consists  of  the  outer  ear,  the  drum,  and  the  labyrinth.  The 
auditory  nerve  transmits  to  the  brain  the  motions  produced  in 
the  ear  by  sound-waves. 


HISTORICAL    SKETCH. 

THE  ancients  knew  that  without  air  we  should  be  plunged 
in  eternal  silence.  "  What  is  the  sound  of  the  voice,"  cried 
Seneca,  "but  the  concussion  of  the  air  by  the  shock  of  the 
tongue?  What  sound  could  be  heard  except  by  the  elasticity  of 
the  aerial  fluid  ?  The  noise  of  horns,  trumpets,  hydraulic  organs, 
is  not  that  explained  by  the  elastic  force  of  the  air?"  Pythag- 
oras, who  lived  in  the  sixth  century  before  Christ,  conceived 
that  the  celestial  spheres  are  separated  from  each  other  by  in- 
tervals corresponding  with  the  relative  lengths  of  strings  arranged 
to  produce  harmonious  tones.  In  his  musical  investigations  he 


HISTOEICAL     SKETCH.  187 

used  a  monochord,  the  original  of  the  sonometer  now  employed 
by  physicists,  and  wished  that  instrument  to  be  engraved  on  his 
tomb.  Pythagoras  held  that  the  musical  intervals  depend  on 
mathematics ;  while  his  great  rival,  Aristoxenes,  claimed  that 
they  should  be  tested  by  the  ear  alone.  The  theories  of  these 
two  philosophers  long  divided  the  attention  of  the  scientific 
world.  The  former  considered  the  subject  from  the  stand-point 
of  Physics,  the  latter  from  that  of  Physiology. 

Many  centuries  elapsed  before  any  marked  advance  was 
made.  Galileo  called  attention  to  the  sonorous  waves  travers- 
ing the  surface  of  a  glass  of  water,  when  the  glass  is  made  to 
vibrate.  He  gave  an  accurate  explanation  of  the  phenomena 
of  resonance,  and  referred  to  the  fact  that  every  pendulum  has 
a  fixed  oscillation  period  of  its  own ;  that  a  succession  of 
properly  timed  small  impulses  may  throw  a  heavy  pendulum 
into  vibration,  and  that  this  may  communicate  vibration  to  a 
second  pendulum  of  the  same  vibration  period.  Galileo  also 
described  the  first  experiment  involving  the  direct  determination 
of  a  vibration  ratio  for  a  known  musical  interval.  He  related 
that  he  was  one  day  engaged  in  scraping  a  brass  plate  with  an 
iron  chisel,  in  order  to  remove  some  spots  from  it,  and  noticed 
that  the  passage  of  the  chisel  across  the  plate  was  sometimes 
accompanied  by  a  shrill  whistling  sound.  On  looking  closely  at 
the  plate,  he  found  that  the  chisel  had  left  on  its  surface  a  long 
row  of  indentations  parallel  to  each  other  and  separated  by 
exactly  equal  intervals.  This  occurred  only  when  a  sound  was 
heard.  It  was  found  that  a  rapid  passage  of  it  gave  rise  to  a 
more  acute  sound,  a  slower  passage  to  a  graver  sound,  and  that 
in  the  former  case  the  indentations  were  closer  together.  After 
many  trials,  two  sets  of  markings  were  obtained,  which  cor- 
responded to  a  pair  of  tones  making  an  exact  fifth  with  each 
other.  The  indentations  were  30  and  45,  respectively,  to  a 
given  length.  Galileo's  inference  from  this  was  exactly  what 
we  now  accept  as  true. 

The  present  century  has  witnessed  a  more  complete  demon- 
stration of  the  laws  of  the  vibrations  of  cords  and  the  general 
principles  of  sound.  In  1822,  Arago,  Gay-Lussac,  and  others 
decided  the  velocity  of  sound  to  be  337  meters  at  10°  C.  Savart 
invented  a  toothed  wheel  by  which  he  determined  the  number 


188  ACOUSTICS. 

of  vibrations  in  a  given  sound  ;  Latour  invented  the  siren,  which 
gave  still  more  accurate  results ;  Colladon  and  Sturm,  by  a 
series  of  experiments  at  Lake  Geneva,  found  the  velocity  of 
sound  in  water  ;  Helmholtz  made  known  the  laws  of  harmonics ; 
Lissajous,  by  means  of  a  mirror  attached  to  the  vibrating  body, 
threw  the  vibrations  on  a  screen  in  a  series  of  curves,  and  so 
rendered  them  visible  ;  while  Tyndall  has  investigated  the  causes 
modifying  the  propagation  of  sound,  as  acoustic  clouds,  fogs, 
etc.,  and  popularized  the  whole  subject  of  acoustics. 


VII. 

•ON  LIGHT. 


THE  sunbeam  comes  to  the  earth  as  simply  motion  of  ether-waves,  yet 
it  is  the  grand  source  of  beauty  and  power.  Its  heat,  light,  and  chemical 
energy  work  every-where  the  wonder  of  life  and  motion.  In  the  growing 
plant,  the  burning  coal,  the  flying  bird,  the  glaring  lightning,  the  blooming 
flower,  the  rushing  engine,  the  roaring  cataract,  the  pattering  rain— we  see 
only  varied  manifestations  of  this  one  protean  energy  which  we  receive 
from  the  sun. 


ANALYSIS  OF  LIGHT. 


1.  PRODUCTION  AND  PROPA- 
GATION OF  LlGHr. 


2.  REFLECTION  OF  LIGHT. 


3.  REFRACTION  OF  LIGHT. 


4.  DECOMPOSITION  OF 
LIGHT. 


6.  OPTICAL  INSTRUMENTS. 


3.  Lenses. 


1.  Definitions. 

2.  Visual  Angle. 

3.  Laws  of  Light. 

4.  Velocity  of  Light. 

5.  Theory  of  Light. 

1.  Definition  and  Law. 

2.  Action  of  Rough  and  Polished  Surfaces. 

f(l.)  Plane. 
(2.)  Concave. 
(3.)  Convex. 

1.  Definition,  and  Illustrations. 

2.  Laws  of  Refraction,  and  Illustrations. 

j  (1.)  Convex. 
1  (2.)  Concave. 

4.  Spherical  Aberration. 

5.  Total  Reflection. 

6.  Mirage. 

r   1.  The  Prismatic  Spectrum. 

2.  Solar  Energy. 

3.  Properties  of  the  Spectrum. 

4.  Interruptions  in  the  Spectrum. 

5.  The  Spectroscope. 

6.  Three  Kinds  of  Spectra. 

7.  Color. 

8.  Complementary  Colors. 

9.  The  Rainbow. 

10.  Chromatic  Aberration. 

r  (1.)  Definition. 
(2.)  By  Double  Refraction. 
(3.)  By  Reflection. 
(4.)  The  Polariscope. 


11.  Polarization. 


1.  Microscope. 

2.  Telescope. 

3.  Opera-glass. 

4.  Projecting  Lantern. 

5.  Camera. 

6.  The  Eye. 

7.  The  Stereoscope. 


OPTICS,  OR  THE  SCIENCE  OF 
LIGHT. 


I.    PRODUCTION    AND    TRANSMISSION    OF    LIGHT. 

1.  Definitions. — A  luminous  body  is  one  that  emits 
light.     A  medium  is  any  substance  through  which 
light  passes.     A  transparent*  body  is  one  that  ob- 
structs light  so  little  that  we  can  see  objects  through 
it.    A  translucent  body  is  one  that  lets  some  light 
pass,  but  not  enough  to  render  objects  visible  through 
it.     An  opaque  body  is  one  that  does  not  transmit 
light.     A  ray  of  light  is  a  single  line  of  light.     A 
pencil  or  beam  of  light  is  a  collection  of  rays,  which 
may  be  parallel,  diverging,  or  converging ;  it  may  be 
traced  in  a  dark  room  into  which  a  sunbeam  is  ad- 
mitted by  the  floating  particles  of  dust  which  reflect 
the  light  to  the  eye. 

2.  The  Visual  Angle  is  the  angle  formed  at  the 
eye  by  rays  coming  from  the  extremities  of  an  ob- 

*  The  terms  transparent  and  opaque  are  relative.  No  substance  is  per- 
fectly transparent,  or  entirely  opaque.  Glass  obstructs  some  light.  Accord- 
ing to  Miller,  7  ft.  of  the  clearest  water  will  arrest  one  half  the  light  which 
falls  upon  it.  While  Young  asserts  that  the  beam  of  the  setting  sun,  pass- 
ing through  200  miles  of  air,  loses  £§§  of  its  fo'rce.  On  the  other  hand,  gold, 
beaten  into  leaf,  becomes  translucent,  transmitting  green  light ;  and  scraped 
horn  is  semi-transparent. 


192  OPTICS. 

ject.  The  angle  AOB.  is  the  angle  of  vision  sub- 
tended by  the  object  AB.  The  size  of  this  angle 
varies  with  the  distance  of  the  body.  AB  and  A'B' 
are  of  the  same  length,  and  yet  the  angle  A  OB'  is 

FIG.  137. 


Variation  of  Visual  Angle  with  Distance. 

smaller  than  AOB,  and  hence  A'B'  will  seem  shorter 
than  AB.  The  distance  and  the  apparent  size  of 
objects  are  intimately  connected,  since  by  experience 
we  have  learned  to  associate  them.  Knowing  the 
distance  of  an  object,  we  immediately  estimate  its 
size  from  the  visual  angle.* 

3.  Laws   of  Light.  —  1.  Light  passes  off  from  a 
luminous  body  equally  in  every  direction.    2.  Light 
travels  through  a  uniform  medium  in  straight  lines. 
3.  The  intensity  of  light  decreases  as  the  square  of 
the  distance  increases. 

4.  The  Velocity  of  Light  has  been  determined  in 
various  ways.     The  following  was  the  first  method: 
The  planet  Jupiter  has  five  moons.    As  these  revolve 
around  the  planet,  they  are  eclipsed  at  regular  inter- 
vals.   In  Fig.  138,  let  J  represent  Jupiter,  e  one  of 
the  moons,  S  the  sun,  and  T  and  t  different  positions 
of  the  earth  in  its  orbit  around  the  sun.    When  the 

*  We  can  vary  the  apparent  size  of  any  body  at  which  we  are  looking  by 
increasing  or  diminishing  this  angle— a  principle  that  will  be  found  of  great 
importance  in  the  formation  of  images  by  mirrors  and  lenses. 


PRODUCTION    AND    TRANSMISSION    OF    LIGHT.     193 

earth  is  at  T,  the  eclipse  occurs  16  min.  and  36  sec. 
earlier  than  at  t.     That  interval  of  time  is  required 


FIG.  138. 

ilillllllllH  


The  Sun,  Earth,  and  Jupiter. 

for  the  light  to  travel  across  the  earth's  orbit,  giving 
a  velocity  of  about  186,000  miles  per  second.* 

5.  Undulatory  Theory  of  Light. — To  account  for 
the  phenomena  presented  by  light  a  substance  is  as- 
sumed to  exist  which  pervades  all  space.  To  this  sub- 
stance the  name  luminiferous  ether,  or,  more  shortly, 
ether,  has  been  given.  In  it  the  heavenly  bodies  are 
immersed,  and  with  it  the  pores  of  all  substances  are 
filled,  so  that  matter  may  be  spoken  of  as  being  "  ether- 
soaked."  The  best  attainable  vacuum  still  contains 
this  substance.  A  luminous  body  sets  in  motion  waves 
of  ether,  which  go  off  in  every  direction.  They  move 
at  the  rate  of  186,000  miles  per  second,  and,  break- 
ing upon  the  eye,  give  the  impression  of  light.  In 
the  wave-motion  of  light,  the  vibrations  are  transverse 
(crosswise)  to  the  direction  of  propagation,  f 

*  This  rate  is  so  great  that  for  all  distances  on  the  earth  it  is  instanta- 
neous. A  sunbeam  would  girt  the  globe  quicker  than  we  can  wink,  if  its 
path  could  be  appropriately  curved. 

t  Thus,  if  we  suppose  a  star  directly  overhead  and  a  ray  of  light  com- 
ing down  to  us,  we  should  conceive  that  some  of  the  particles  which  com- 


194  OPTICS. 

II.    REFLECTION   OF   LIGHT. 

1.  Definition. — Light  falling  on  a  surface  is  divided 
into  two  portions.     One  enters  the  body;    the  other 
is  reflected*  according  to  the  familiar  law  of  Motion 
and  of  Sound:    The  angle  of  incidence  is  equal  to 
that  of  reflection. 

2.  Action    of   Rough    and    Polished    Surfaces.  — 

When  the  surface  is  rough,  the  numerous  little  ele- 
vations scatter  the  reflected  rays  in  every  direction, 
forming  diffused  light.  Such  a  body  can  be  seen 
from  any  point.  When  the  surface  is  polished,  the 
rays  are  uniformly  reflected  in  particular  directions, 
and  may  bring  to  us  the  images  of  other  objects. 
We  thus  see  non-luminous  objects  by  irregularly-re- 
flected (diffused)  light,  and  images  of  objects  by  reg- 
ularly-reflected light,  f 

3.  Mirrors. — All  highly-reflecting  surfaces  are  mir- 
rors.    These  are  of  three  kinds— -plane,  concave,  and 
convex.    The  first  has  a  flat  surface ;  the  second,  one 

pose  the  waves  are  vibrating  E.  and  W.,  others  N.  and  S.,  and  others  to- 
ward all  other  possible  points  of  the  compass  in  succession. 

*  The  amount  of  light  reflected  varies  with  the  angle  at  which  light 
falls.  Thus,  if  we  look  at  the  images  of  objects  in  still  water,  we  notice 
that  those  near  us  are  not  so  distinct  as  those  on  the  opposite  bank.  The 
rays  from  the  latter  striking  the  water  more  obliquely,  are  more  perfectly 
reflected  to  the  eye.— Fill  any  dark-colored  pail  with  water  tinted  with 
bluing  or  red  ink.  The  color  will  be  quite  invisible  to  a  spectator  at  a  little 
distance.  Now  insert  in  the  water  a  plate.  This  will  reflect  the  transmitted 
light  and  reveal  the  hue  of  the  water. 

t  The  most  perfectly  polished  substance,  however,  diffuses  some  light- 
enough  to  enable  us  to  trace  its  surface ;  were  it  not  so,  we  should  not  be 
aware  of  its  existence.  The  deception  of  a  large  plate-glass  mirror  is  often 
nearly  complete ;  but  dust  or  vapor,  increasing  the  irregular  reflection,  will 
bring  its  surface  to  view. 


REFLECTION     OF     LIGHT.  195 

like  the  inner  surface  of  a  hollow  globe ;  the  third, 
like  part  of  its  outer  surface.  The  general  principle 
of  mirrors  is  that  the  image  is  seen  in  the  direction 
of  the  reflected  ray  as  it  enters  the  eye. 

(1.)  PLANE  MIRRORS. —  Rays  of  light  retain  their 
relative  direction  after  reflection  from  a  plane  sur- 
face.* While  standing  before  a  plane  mirror,  one 
sees  his  image  erect  and  of  the  same  size  as  him- 
self. It  is,  however,  reversed  right  and  left. 

Why  the  image  is  as  far  behind  the  mirror  as  the 
object  is  in  front.  Let  AB 

FIG.  139. 

be  an  arrow  held  in  front 

of  the  mirror  MN.    Rays 

of  light  from  the  point  A 

striking  upon  the  mirror 

at    Cj   are    reflected,    and 

enter  the   eye  as  if  they 

came  from  a.    Rays  from 

B  seem  to  come  from  6. 

Since    the    image    is   seen   in    the    direction   of   the 

reflected  rays,  it  appears  at  a&,  a   point  which  can 

easily   be    proved  to  be   as   far    behind  MN  as   the 

arrow  is  in  front  of  it.     Such  an  image  is  called  a 

*  The  perpendiculars  are  not  given  in  the  figures  of  the  book,  as  the 
pupil  at  recitation  should  draw  all  the  cuts  on  the  blackboard,  erect  the  perpendiculars, 
and  demonstrate  the  location  of  the  reflected  ray.  It  will  aid  in  drawing  the  per- 
pendicular to  a  convex  or  concave  surface,  to  remember  that  it  is  a  radius 
of  the  sphere  of  which  the  mirror  forms  a  part.  A  book  held  in  various 
positions  before  a  looking-glass  illustrates  the  action  of  plane  mirrors.  A 
beam  of  light  admitted  into  a  dark  room  and  reflected  from  a  mirror  will 
show  that  the  angles  of  incidence  and  reflection  are  in  the  same  plane. 
Many  of  the  grotesque  effects  of  concave  and  convex  mirrors  may  be  seen 
on  the  inner  and  outer  surfaces  of  a  bright  spoon,  call-bell,  or  metal  cup 
(see  "Mayer  &  Barnard's  Light"  for  inexpensive  experiments). 


196  OPTICS. 

virtual  one,  as  it  has  no  real  existence  apart  from 
the  observer's  eye. 

Why  we  can  see  several  images  of  an- object  in  a 
mirror.    Metallic  mirrors  form  only  a  single  image. 
FIG  140  ^   however,   we   look   obliquely   at 

the  image  of  a  candle  in  a  looking- 
glass,  we  shall  see  several  images, 
the  first  feeble,  the  next  bright,  and 
the  others  diminishing  in  intensity. 
The  ray  from  A  is  in  part  reflected 
to  the  eye  from  the  glass  at  &,  and 
gives  rise  to  the  image  a ;  the  re- 
mainder passes  on  and  is  reflected  from  the  metallic 
surface  at  c,  and  coming  to  the  eye  forms  a  second 
image  a'.  The  ray  cd,  when  leaving  the  glass  at 
d,  loses  a  part,  which  is  reflected  back  to  form  a 
third  image.  This  ray  in  turn  is  divided  to  form 
a  fourth,  and  so  on. 

If  two  mirrors  are  arranged  as  in  Fig.  141,  three 
images  of  a  candle  may  be  seen.  (Let  the  pupil 
trace  the  formation  of  each  by  the  diagram  of  Fig. 
142.)  To  vary  the  experiment,  hold  the  mirrors  to- 
gether like  the  covers  of  a  book  placed  on  end,  and 
put  the  candle  between  them  on  the  table,  opening 
and  shutting  the  mirror-cover  so  as  to  vary  the 
angle ;  or  hold  the  mirrors  parallel  to  each  other 
with  the  light  between  them.  When  the  mirrors  are 
inclined  at  90°,  three  images  are  formed ;  at  60°, 
five  images;  and  at  45°,  seven  images.  As  the 
angle  increases,  the  number  diminishes.  The  images 
are  upon  the  circumference  of  a  circle  whose  center 


REFLECTION     OF     LIGHT. 


197 


is  on  a  line  in  which  the  reflecting  surfaces  would  in- 
tersect if  produced.   Where  the  mirrors  are  parallel  the 


FIG.  141. 


PIG.  142. 


Multiple  Reflection. 

images  are  in  a  straight  line.    They  become  dimmer 

as  they  recede,  light  being 

lost  at  each  reflection. — The 

Kaleidoscope  contains  three 

mirrors  set  at  an  angle   of 

60°.    Small  bits  of  colored 

glass  at  one  end  reflect  to 

the  eye  at  the  other  multi-    ^ 

pie  images  which  change  in     7:    / 

varying  patterns  as  the  tube     '*'x 

is  revolved. 

Images  seen  in  water  are  symmetrical,  but  in- 
verted. The  reason  of  this  can  be  understood  by 
holding  an  object  in  front  of  a  horizontal  looking- 
glass  and  noticing  the  angle  at  which  the  rays  must 
strike  the  surface  in  order  to  be  reflected  to  the  eye. 


198 


OPTICS. 


Fro.  148. 


When  the  moon  is  high  in  the  heavens,  we  see  the 
image  in  the  water  at  only  one  spot,  while  the  rest 

of  the  surface  ap- 
pears dark.  The 
light  falls  upon  all 
parts,  but  each 
ray  is  reflected 
from  only  one 
point  at  the  proper 
angle  to  reach  the 
eye.  Each  ob- 
server sees  the 
image  at  a  differ- 
ent place.  When 
the  surface  of  the 
water  is  ruffled,  a 
tremulous  line  of 
light  is  reflected 
from  the  side  of 
each  tiny  wave 

that  is  turned  toward  us.  As  every  little  billow 
rises,  it  flashes  a  gleam  of  light  to  our  eyes,  and 
then  sinking,  comes  up  beyond,  to  reflect  another 
ray. 

(2.)  A  CONCAVE  MIRROR  tends  to  collect  the  rays 
of  light  to  a  focus.  In  Fig.  144,  O  is  the  center  of 
curvature,  i.e.,  the  center  of  the  hollow  sphere  of 
which  the  mirror  is  a  part ;  V  is  the  vertex,  or  mid- 
dle of  the  mirror;  F  is  the  principal  focus;  it  is 
half-way  between  C  and  V.  Any  ray  which  passes 
through  G  is  an  axis ;  it  is  called  the  principal  axis 


REFLECTION     OF     LIGHT.  199 

if  it  pass  also  through  V,  otherwise  it  is  a  secondary 
axis.  All  axial  rays  are  reflected  back  upon  their 
own  paths.  All 

rays    parallel    to  FlG>  144< 

the  principal  axis 
cross  at  the  prin- 
cipal focus  after 
reflection,  and 

conversely    all     ^     > 

rays  which  pass 

through  the  prin-  Parallel  Rays  Reflected  to  the  Focus. 

cipal    focus    will 

be  reflected  parallel  to  the  principal  axis.*  An  image 
is  real  if  the  rays  after  reflection  cross  before  reach- 
ing the  eye ;  it  will  appear  to  be  at  the  crossing 
point.  Otherwise,  the  image  is  virtual. 

Images  formed  ~by  Concave  Mirrors. — In  a  dark 
room  place  a  candle  (PQ,  Fig.  145)  in  front  of.  a 
concave  mirror  at  some  distance  beyond  its  center 
of  curvature.  A  small  inverted  image  of  it  will  ap- 
pear to  be  suspended  in  mid-air  near  its  focus.  It  is 
easy  to  determine  the  position  of  this  image.  From 
P  draw  an  axial  ray  through  (7;  it  will  be  reflected 
back  on  its  own  path,  hence  the  image  of  P  must  be 

*  These  statements  are  approximately  true  only  for  mirrors  of  slight 
curvature,  where  the  angle  M CW,  or  angular  aperture,  does  not  exceed  8°  or 
10=.  When  greater,  the  rays  reflected  near  the  edge  of  the  mirror  meet  the 
principal  axis  VC,  nearer  the  mirror  than  F.  This  is  called  the  aberration 
of  the  mirror.  The  reflected  rays  will  then  cross  at  points  in  a  curved 
surface  called  a  caustic.  A  section  of  such  a  curve  can  be  seen  when  the 
light  of  a  candle  is  reflected  from  the  inside  of  a  cup  partly  full  of  milk. 
All  of  these  phenomena  can  be  proved  mathematically  to  be  necessary  con- 
sequences of  the  one  law,  that  the  angles  of  incidence  and  reflection  are 
equal. 


200  OPTICS. 

on  this  line.  From  P  draw  also  a  ray,  Pa,  parallel 
to  the  principal  axis ;  after  reflection  it  will  pass 
through  the  focus,  F,  and  cross  the  secondary  axis 
at  P',  which  is  hence  the  position  of  the  image  of 
P.  In  like  manner  we  may  determine  Q'.  If  a  piece 
of  thin  white  paper  or  roughened  glass  be  put  at 
P'Q'j  the  light  will  seem  to  come  from  it  since  the 
rays  cross  here. 

Bring  the  candle  closer  to  the  mirror.  The  image 
will  grow  larger  and  move  from  F  toward  C.  When 
the  candle  reaches  C,  the  image  will  fall  upon  it  and 

Fie.  145. 


Inverted  Real  Image  of  a  Candle. 

just  cover  it.  When  it  reaches  P'Q',  the  image  will 
have  receded  to  PQ,  and  in  every  case  the  ratio  of 
the  lengths  of  candle  and  image  will  be  the  same  as 
the  ratio  of  their  distances  from  C.  P  and  P'  are 
called  conjugate  points;  for  they  are  so  related  that 
an  object  placed  at  one  of  them  will  be  imaged  at 
the  other. 

If  the  candle  be  moved  toward  F,  the  image  in- 
creases in  size  and  recedes  from  C.  At  J^the  image 
vanishes.  Passing  F  the  image  again  appears,  but  now 
as  if  it  were  behind  the  mirror,  larger  than  the  object 


REFLECTION     OF     LIGHT. 


201 


FIG.  146. 


and  erect.    Let  the  student  trace  the  rays,  as  shown 
in  Fig.  146,  and  satisfy  himself  that  they  can  never 
cross    after     reflec- 
tion.   The  image  is 
hence    virtual;   it 
can  not  be   caught 
on    a    screen ;     but 
its  apparent  length 
is  as  much  greater 


Virtual  Image  in  a  Concave  Mirror. 


than    that    of    the 
candle  as  its  appar- 
ent distance  from  O  is  greater.    Moreover,  it  appears 
erect,  and   not  inverted  like  the  real  image  of  the 
more  distant  candle. 

(3.)  CONVEX  MIRRORS. — Let  the  position  of  a  can- 
dle be  varied  in  front  of  a  convex  mirror.  It  will  be 
found  that  the  image  is  always  virtual,  erect,  and 

FIG.  147. 


Virtual  Image  in  a  Convex  Mirror. 

smaller  than  the  candle.     Parallel  rays  are  made  to 
diverge  after  reflection,  as  if  they  had  come  from  a 


202 


OPTICS. 


point  within  the  sphere,  half-way  between  its  surface 
and  center.  The  image  of  P  is  at  the  crossing  point 
of  the  axial  ray  from  P  and  the  backward  prolonga- 
tion of  the  ray  from  P  which  was  parallel  before  re- 
flection. The  student  can  easily  trace  the  rays  and 
determine  the  position  of  the  image. 


III.    REFRACTION   OF   LIGHT. 

1.  Definition. — When  a  ray  of  light  passes  ob- 
liquely from  one  medium  to  another  of  different 
density,  it  is  refracted  or  bent  out  of  its  course.— 
Examples:  A  spoon  in  clear  tea  appears  bent. — An 
oar  dipping  in  still  water  seems  broken  at  the  point 
where  it  enters  the  water.* — Put  a  cent  in  a  bowl. 
Standing  where  you  can  not  see  the  coin,  let  another 

*  In  Pig.  148  is  shown  a  stick  sunk  till  the  end  is  at  the  bottom  of 

the  water.  Bays  of  light  from 
this  end  are  bent  as  they 
emerge  from  the  liquid  and 
reach  the  eye  as  if  they  had 
come  from  a  point  consider- 
ably higher.  The  entire  bot- 
tom, therefore,  seems  lifted  up. 
Hence,  water  is  always  deeper 
than  it  appears.  Look  oblique- 
ly into  a  pail  of  water,  then 
place  your  finger  on  the  out- 
side where  the  bottom  seems 
to  be ;  you  will  be  surprised  to 
find  the  real  bottom  is  several 
inches  below.  —  Fill  a  glass  dish 
with  water,  and,  darkening  the 
windows,  let  a  sunbeam  fall 

upon  the  surface.     The  ray  will  bend  as  it  enters.    Dust  scattered  through 

the  air  will  make  the  beam  distinct. 


Apparent  breaking  of  a  Stick  in  Water. 


REFRACTION     OF     LIGHT. 


203 


FIG.  149. 


person  pour  water  into  the  vessel,  when  the  coin  will 
be  lifted  into  view.  To  understand  the  apparent 
change  of  position,  remember  that  the 
object  is  seen  in  the  direction  of  the  re- 
fracted ray  as  it  enters  the  eye.  Let  L, 
Fig.  149,  be  a  body  beneath  the  water. 
A  ray,  LA,  coming  to  the  surface,  is 
bent  away  from  the  vertical,  LK,  and 
strikes  the  eye  as  if  it  came  from  L. 
The  object  will  therefore  apparently  be  elevated 
above  its  true  place. 

2.  Laws  of  Refraction. — From  any  point,  A,  Fig. 
150,  let  a  beam  of  light,  AS,  pass  through  air  and 
meet  a  denser  transparent  medium  at  B,  such  as 

water  or  glass.  At  this 
point  let  a  line,  DE,  be 
drawn  perpendicular  to 
the  surface.  Then  some 
of  the  light  will  be  re- 
flected at  B,  the  angle 
of  reflection  DBF  being 
equal  to  the  angle  of  in- 
cidence, DBA.  A  little 
of  it  will  be  absorbed 
and  changed  into  heat. 
The  rest  will  be  trans- 
mitted, but  its  direction 
changed  to  BC.  This  apparent  breaking  of  the  ray 
is  called  refraction,  and  the  angle  EEC,  which  is  less 
than  DBA,  is  the  angle  of  refraction.  If  the  source 
of  light  were  at  C,  its  direction  on  emerging  at  B 


Reflection  and  Refraction. 


204 


OPTICS. 


would  be  BA.    The  angle  of  refraction  now  is  DBA 
and  the  angle  of  incidence  is  EBC.    Hence, 

I.  In  passing  into  a  denser  medium,  the  ray  is 
bent  toward  the  perpendicular. 

II.  In  passing  into   a   rarer  medium,  the  ray  is 
bent  from  the  perpendicular.* 

ILLUSTRATIONS. — Path  of  rays  through  a  window- 
glass. — When  a  ray  enters  a  win- 
dow-glass, it  is  refracted  toward 
the  perpendicular  (1st  law),  and, 
on  leaving,  is  refracted  equally 
from  the  perpendicular  (2d  law). 
The  general  direction  of  objects 
is  therefore  unchanged.  A  poor 
quality  of  glass  produces  distor- 
tion by  its  unequal  density  and 
uneven  surface. 
Path  of  rays  through  a  prism. — A  ray  of  light,  on 

entering   and   on 

leaving   a  glass    *^ 

prism,  is  refracted. 

The  inclination  of 

the    sides    causes 

the  ray  to  be  bent 

twice  in  the  same 

direction.     The 

candle,    L,    will 

therefore   appear  to  be   in   the   direction   of  r. 


Passage  of  a  Ray  through 
Window-glass. 


Passage  of  a  Ray  through  a  Prism, 


*  Both  the  incident  and  the  refracted  ray  lie  in  the  same  plane  as  the 
normal  (perpendicular).  The  ratio  hetween  the  nines  of  the  angles  of  inci- 
dence and  refraction  is  termed  the  index  of  refraction.  It  varies  with  the 
media.— Example;  From  air  to  water  it  is  |  and  from  air  to  glass  f. 


REFRACTION     OF     LIGHT. 


205 


3.  Lenses. — A  lens  is  a  transparent  body,  with  at 
least  one  curved  surface.  There  are  two  general 
classes  of  lenses,  concave  and  convex*  (See  Mg.  153.) 


FIG.  153. 


Magnifying  Lenses. 


Diminishing  Lenses. 


(1.)  THE  BI-CONVEX  LENS  has  two  convex  surfaces. 
Its  action  on  light  is  like  that  of  a  concave  mirror. 
A  ray,  X,  striking  perpendicularly,  is  not  refracted. 


FIG.  154. 


Bi-convex  Lens. 

The  parallel  rays,  M,  L,  etc.,  are  refracted  both  on 
entering  and  on  leaving  the  lens,  and  are  converged 
at  Fj  the  principal  focus.  \  If  a  luminous  point  be 
placed  at  F,  its  rays  will  emerge  parallel. 

*  Forms  of  lenses:  M,  double-convex;  N,  plano-convex;  0,  meniscus 
(crescent) ;  P,  double-concave ;  Q,  plano-concave ;  E,  concavo-convex.  The 
first  three  are  styled  magnifiers,  and  the  second,  diminishers. 

t  The  convex  lens  is  sometimes  termed  a  burning-glass,  from  the  fact  that, 
like  a  concave  mirror,  it  collects  and  brings  to  a  focus  the  rays  of  the  sun ; 
and  combustible  substances  placed  at  this  focus  may  be  burned.  Even 
glass  globes  of  water,  such  as  are  used  for  gold-fishes  or  in  the  windows  of 
drug  stores,  may  fire  adjacent  objects. 


206  OPTICS. 

Construction  of  Images. — There  is  a  point,  called 
the  optical  center  of  the  lens,  through  which  the 
passing  -ray  does  not  change  its  general  direction 
after  emergence.  These  are  called  axial  rays.  The 
principal  axis  passes  not  only  through  the  optical 
center  (C,  Fig.  155),  but  also  through  the  principal 
focus,  F.  All  rays  parallel  to  it  are  so  refracted  as 
to  pass  through  F.  Let  a  candle,  PQ,  be  placed  in 
front  of  a  bi-convex  lens,*  at  a  distance  of  ten  or 


p' 

Formation  of  an  Image  with  a  Bi-convex  Lens. 

twelve  feet.  An  axial  ray,  PC,  continues  its  path 
unchanged.  A  parallel  ray,  Pa,  will  after  refraction 
pass  through  F.  Where  this  cuts  the  axial  ray  at  P', 
the  image  of  P  is  found.  In  like  manner  Q'  is  found 
as  the  point  conjugate  to  Q.  The  image,  P'Q',  is 
real,  and  may  be  caught  on  a  screen.  ,It  is  inverted, 
and  as  much  smaller  than  the  object  as  its  distance 
from  the  optical  center  is  less. 

As  the  candle  is  made  to  approach  the  lens  on 
one  side,  the  image  recedes  on'  the  other.  When 

*  An  ordinary  magnifying  hand-glass,  such  as  is  often  used  in  looking 
at  photographs,  or  even  a  spectacle-lens  used  by  an  aged  person  in  reading, 
•will  be  sufficient  for  these  experiments. 


REFRACTION     OF     LIGHT. 


207 


Fie.  156. 


brought  nearly  to  F,  the  image  on  the  other  side 
grows  very  large  and  distant.  When  it  arrives  with- 
in the  focal  dis- 
tance, FC,  the 
image  suddenly 

appears    on    the  /      -  jc^l 

same  side  with 
the  object,  erect, 
and  as  much 
larger  as  its  ap- 
parent distance 
from  O  is  greater.  This  image  is  virtual.  The 
student  can  determine  this  by  tracing  the  rays  in 
Fig.  156. 

(2.)  THE  BI-CONCAVE  LENS  has  two  concave  sur- 
faces. Its  action  on  light  is  like  that  of  a  convex 
mirror.  Thus,  diverging  rays  from  L  (Fig.  157)  are 

FIG.  157. 


Virtual  Image  with  Convex  Lens. 


Bi-concave  Lens. 

rendered  more  diverging,  and,  to  an  eye  which  re- 
ceives the  rays  MN,  the  candle  would  seem  to  be  at 
Z,  where  the  image  is  seen.* 

*  Unscrew  the  eye-piece  from  an  opera-glass;  it  serves  well  for  experi- 
ments with  concave  lenses.  Unscrew  the  glass  at  the  other  end ;  it  serve* 
for  those  with  convex  lenses. 


208 


OPTICS. 


The  image  formed  ~by  a  concave  lens,  like  that  of 
a  convex  mirror,  is  virtual,  erect,  and  diminished  in 


FIG.  158. 


Formation  of  an  Image  with  a  Concave  Lens. 

size  (Fig.  158).  Let  the  student  determine  this  by 
tracing  the  rays  in  Fig.  158,  in  which  the  arrow  PQ 
is  the  object  and  P'Q'  the  image.* 

4.  Spherical  Aberration  of  Lenses. — Parallel  rays 
falling   on  a   lens   whose   surface   is   like   that   of  a 
sphere  are  not  all  refracted  to  a  single  focus.    Those 
which  pass  through  marginal  parts  of  the  lens  are 
collected  to  a  focus  nearer  than  that  to  which  the 
central  rays  are  collected.    With  a  single  lens,  there- 
fore, it  is  not  easy  to  secure  perfect  distinctness  of 
image. 

5.  Total    Reflection.  —  In   passing  from    a    dense 
into  a  rare  medium,  the  angle  of  refraction  is  greater 
than  the  angle  of  incidence.     It  can  not  exceed  90°, 
for  then  the  ray  would  cease  to  emerge.     The  angle 

*  Remember  that  parallel  rays,  Pa  and  $>,  become  divergent  after  re- 
fraction, as  if  they  had  come  from  a  focus,  F,  on  the  same  side  of  the  lens. 
The  images  of  P  and  Q  must  hence  be  found  on  the  backward  prolongations 
of  these  emergent  rays.  Axial  rays  are  drawn  from  P  and  Q  to  C. 


REFRACTION     OF     LIGHT. 


209 


Flo.  159. 


of  incidence  for  which  the  emergent  ray  would  make 
an  angle  of  90°  with  the  per- 
pendicular is  called  the  critical 
angle.  The  light  is  then  totally 
reflected  in  the  dense  medium 
as  if  its  surface  were  the  most 
perfect  of  mirrors.  When  we 
look  obliquely  into  a  pond,  we 
can  not  see  the  bottom,  because 
the  rays  of  light  from  below 
are  reflected  downward  at  the 
surface  of  the  wate^r.  Hold  a 
glass  of  water  above  the  level 
of  the  eye,  and  the  upper  part 
will  gleam  like  burnished  silver.* 
Thus  the  internal  surface  of  a 
transparent  body  becomes  a 

mirror  Total  Internal  Reflection. 

6.  Mirage. — Over  the  heated  deserts  of  Arabia 
and  Africa  the  traveler  sometimes  sees  a  shimmer- 
ing expanse,  as  if  a  quiet  lake  were  in  the  distance, 
in  which  the  scattered  trees  are  mirrored  upside 
down.  The  layer  of  air  close  to  the  uniformly  heated 
sand  is  less  dense  than  the  cooler  air  above.  A  ray 
coming  obliquely  downward  from  a  tree-top  may  be 
so  bent  from  its  first  direction  by  passing  through 

*  Place  a  bright  spoon  in  the  glass  and  notice  its  image  reflected  from 
the  surface  of  the  water.  Turn  the  spoon  about  in  the  glass  and,  changing 
the  angle  of  observation,  notice  the  effect.  The  real  handle  may  apparently 
be  attached  to  the  image  in  the  water.  The  spoon  will  soon  be  covered 
with  bubbles  of  air  shining,  like  pearls,  from  total  reflection.  This  shows 
also  the  presence  of  air  in  water  and  the  adhesion  of  gases  to  solids. 


210 


OPTICS. 


these  different  media  as  to  be  sent  obliquely  upward 
to  the  eye.  The  low  warm  layer  of  air  acts  like  a 
totally  reflecting  mirror,  and  inverted  images  are 
dimly  seen  amid  the  bright  light  along  the  horizon. 


FIG.  160. 


In  Fig.  160,  rays  of  light  from  a  clump  of  trees  are 
refracted  more  and  more  until  finally  they  are  bent 
upward  from  a  layer  at  a,  and  enter  the  eye  of  the 
Arab  as  if  they  came  from  the  surface  of  a  quiet 
lake. 


IV.    COMPOSITION   OF   LIGHT. 

1.  The  Prismatic  Spectrum. — When  a  sunbeam 
is  received  through  a  narrow  slit  and  transmitted 
through  a  prism,  properly  placed,  the  ray  is  not  only 
bent  from  its  course,  but  is  also  spread  out  into  a 
band  of  rainbow  colors — the  solar  spectrum.  This 
includes  a  multitude  of  tints  grading  imperceptibly 


COMPOSITION     OF     LIGHT. 


211 


from  one  to  another.  The  most  prominent  are  violet, 
indigo,  blue,  green,  yellow,  orange,  red*  If  we  re- 
ceive the  spectrum  on  a  concave  mirror  or  pass  it 
through  a  convex  lens  appropriately  adjusted  in 
position,  these  colors  may  be  recombined  so  as  to 
form  a  white  band.  We  therefore  conclude  that 

FIG.  161. 


The  Prismatic  Spectrum. 

white  light  is  made  up  of  these  many  tints.  Be- 
cause each  has  its  own  separate  index  of  refraction 
(see  p.  204,  foot-note)  when  passing  through  the 
same  prism,  this  refracts  them  unequally.  The  de- 
viation of  the  violet  is  the  greatest,  and  that  of  the 
red  is  the  least,  for  the  visible  rays  of  the  spectrum. 

*  Notice  that  the  initial  letters  spell  the  mnemonic  word, 


212  OPTICS. 

2.  Solar  Energy. — What  we  receive  from  the  Sun 
is  called  SOLAE  ENERGY.    It  reaches  us  in  tiny  waves, 
the  longest  of  which  are  so  minute  that   8,000   of 
them  in   succession  would  be  required  to  cover  an 
inch.     The  shortest  that    have   been    measured   are 
about  a  tenth  as  long,  or  -50  ibr  of    an    inch.      The 
longer  ones  are  manifested  largely  as  heat  ;  some  of 
the  intermediate   ones  as  light,   and  the  shortest  as 
chemical  energy.    All  these  waves  come  mixed  to- 
gether in   the   sunbeam.    The    prism   changes    their 
direction,  and  arranges  them  in  the  order  of  their 
wave-lengths. 

3.  Properties  of  the  Spectrum.— Only  a  small  pr^rt 
of  the  solar  spectrum  is  perceptible  to  the  eye.    Be- 
yond the  violet  all  is  dark,  but  by  employing  a  pho- 
tographic plate  this  invisible  part  of  the   spectrum 
may  be  photographed.    By  means  of  Langley's  bo- 
lometer (page  347),  an  instrument  sensitive  to  varia- 
tions   of   temperature,  we    may  explore    the    region 
beyond  the  red,  and  show  that  the  spectrum  continues 
also  in  this  direction.    Solar  energy  appears  thus  to 
have  been  separated  by  the  prism  into  parts  having 
different  properties     Part  producing  the  sensation  of 
light,  another  part  lying  beyond  the  red  manifesting 
itself  mainly  as  heat,  and  still  another  part  beyond 
the  violet  distinguished  by  its  chemical  effects.    This 
difference  in  properties  is,   however,    only  apparent. 
There  is  physically  no  distinction  between  the  rays 
occupying  different  parts  of  the  spectrum  except  a 
difference  of  wave-length.    A  ray  belonging  to  any 
part  of  the  visible  spectrum  shows  all  three  properties. 


COMPOSITION    OF     LIGHT.  213 

It  affects  the  eye  with,  the  sensation  of  color,  the 
photographic  plate  in  producing  chemical  change, 
and  the  bolometer  in  heating  the  filament  of  platinum. 

4.  Interruptions    in    the    Spectrum. — When    the 
spectrum  of  the  sun  is  carefully  examined,  it  is  found 
that  there  are  numerous  breaks  in  both  the  visible 
and  invisible  parts.    Numerous  black  lines,  parallel 
to  the  slit  that  transmits  the  light,  may  be  detected 
in  the  visible   part.     The  more   prominent  of  these 
have  been  named.    Thus,  the  A,  B,  and  O  lines  are 
in  the  red ;  the  D  line  in  the  yellow ;  the  E  line  in  the 
green ;  the  F  line  in  the  blue ;  the  Q-  and  H  lines  in 
the  violet.    The  interruptions  in  the  invisible  portion 
are  far  broader,  becoming  bands  rather  than  narrow 
lines. 

5.  The  Spectroscope  is  an  instrument  used  for  the 
production  and  examination  of  spectra.    It  consists  of 
one  or  mere  prisms  for  producing  the  spectrum,  and  a 
telescope  for  examining  it.    From  the  source  of  light, 
Q-j  Fig.  162,  the  rays  pass  through  an  adjustable  slit, 
and  are  made  parallel  by  the  lens  in  the  tube,  _B,  before 
passing  through  the  prism,  P.    The  spectrum  is  seen 
through  the  telescope  A.     The  tube,   (7,  has  at  one 
end  a  scale  on  glass  through  which  passes  the  light 
from  a  candle  or  coal-gas  jet  at  F.    This  is  reflected 
from  the  surface  of  the  prism  into  the  telescope  At 
where  an  image  of  the  scale  is  seen  alongside  of  the 
spectrum.    Each  part  of  the  spectrum  can  thus  be 
distinguished    by    its   own    scale    number.     Instead 
of  a  single  prism,  often  a  train  of  prisms  is  used, 


214 


OPTICS. 


thus    widening    the    spectrum    and    diminishing    its 
brightness.* 


FIG.  162. 


The  Spectroscope. 


6.  Three  Kinds  of  Spectra. — If  in  the  spectro- 
scope we  examine  the  light  of  a  glowing  thin  gas 
or  vapor,  its  spectrum  is  seen  to  consist  of  one  or 
more  bright  lines  only.  Thus  burning  sodium  gives 

*  On  the  uses  of  the  spectroscope,  examine  "  New  Astronomy,"  p.  258, 
and  "  Popular  Chemistry,"  p.  147.  In  the  former,  opposite  p.  258,  is  a  colored 
illustration  of  the  spectra.— The  dark  lines  which  cross  the  solar  spectrum 
are  known  as  Fraunhofer''s  lines,  being  so  named  in  honor  of  the  physicist  who 
first  carefully  studied  and  mapped  them.  The  spectroscope  affords  an  un- 
rivaled mode  of  analysis.  No  chemical  test  is  so  delicate.  Strike  together 
two  books  near  the  light  at  the  slit  of  the  spectroscope,  and  the  dust  blown 
into  the  flame  will  contain  enough  sodium  (the  basis  of  common  salt)  to 
cause  the  yellow  D  lines— its  test— to  flash  out  distinctly.  A  very  effective 
spectroscope  may  be  contrived  thus :  Cut  a  slit  not  over  ^  inch  wide  and 
2  inches  long  in  a  piece  of  tin-foil,  and  gum  it  on  a  pane  of  glass.  Hold 
this  before  a  flame  and  look  at  it  through  a  prism. 


COMPOSITION     OF     LIGHT.  215 

a  pair  of  brilliant  yellow  lines  close  together ;  zinc 
vapor,  a  number  of  lines  among  which  the  blue  are 
very  prominent ;  and  strontium,  a  number  among 
which  the  red  are  conspicuous.  Each  element,  if 
made  gaseous,  can  be  thus  recognized  by  its  spec- 
trum. 

If  the  light  from  a  glowing  solid  be  examined,  its 
spectrum  is  found  to  be  continuous,  giving  all  the 
colors  without  interruption.  White-hot  lime,  or  the 
particles  of  carbon  in  a  common  candle-flame,  furnish 
a  continuous  spectrum. 

The  bright  lines  given  by  a-  glowing  gas  may  be 
made  to  broaden  into  bands,  and  these  finally  to  be- 
come joined  into  a  nearly  continuous  spectrum  by 
subjecting  the  gas  to  very  great  pressure,  and  thus 
making  it  very  dense.  There  is  no  sharp  distinction 
between  line  spectra  and  continuous  spectra. 

The  interrupted  spectrum  is  that  given  by  the 
sun  and  stars.  It  may  be  produced  to  a  limited  ex- 
tent by  interposing  a  glowing  vapor,  like  that  of 
sodium,  between  the  spectroscope  and  a  white-hot 
solid,  like  lime.  It  is  believed  that  the  body  of  the 
sun  and  of  each  of  the  stars  is  made  up  of  very 
dense  glowing  matter,  which  is  surrounded  by  less 
hot  vapors.  These  absorb  some  of  the  light  from 
within,  and  thus  produce  the  interruptions  observed 
in  the  spectra  of  the  sun  and  stars.  Moreover,  it  has 
been  proved  that  each  gas  or  vapor  absorbs  the  same 
waves  as  those  given  out  by  itself  in  glowing.  By 
comparing  the  bright  lines  of  a  known  gas  or  vapor 
with  the  dark  lines  in  the  sun  or  star  spectrum,  it 


216  OPTICS. 

becomes  possible   to   determine   whether   this  vapor 
exists  in  the  atmosphere  of  the  sun  or  star. 

7.  Color.  —  If  a   piece   of  pure   red  paper  is  put 
against  the  successive  parts  of  the  spectrum  on  a 
screen,  it  will  look  red  only  when  in  the  red  part, 
but  dark  gray  or  black  in  the  other  parts.    It  reflects 
red  light  and  absorbs  the  other  tints.     Color  is  anal- 
ogous to  pitch,  violet  corresponding  to  the  high  and 
red  to  the  low  sounds  in  music.     Intensity  of  color, 
as  of  sound,  depends  on  the  amplitude  of  the  vibra- 
tions.    When  a  body  absorbs  all  the  colors  of  the 
spectrum  except  blue,  but  reflects  that  to  the  eye, 
we  call  it  a  blue  body ;  when  it  absorbs  all  but  green, 
we  call  it  a  green  body.*     Red  glass  has  the  power 
of  absorbing  all  except  the  red  rays,  which  it  trans- 
mits.   When  a  substance  reflects  all  the  colors  to  the 
eye,  it  seems  to  us  white.    If  it  absorbs  all  the  colors, 
it  is  black.     Thus  color  is  not  an  inherent  property 
of  objects.f    In  darkness  all  things  are  colorless. 

8.  Complementary  Colors. — Two  colors,  which  by 
their  mixture  produce  white  light,  are  termed  com- 

*  Some  eyes  are  blind  to  certain  colors,  as  some  ears  are  deaf  to  certain 
sounds.  "  Color-blindness "  generally  exists  as  to  red.  Such  a  person  can 
not  by  the  color  distinguish  ripe  cherries  from  green  ones.  Doubtless  rail- 
way accidents  have  occurred  through  this  inability  to  apprehend  signals. 
Dr.  Mitchell  mentions  a  naval  officer  who  chose  a  blue  coat  and  red  waist- 
coat, believing  them  of  the  same  color ;  a  tailor  who  mended  a  black  silk 
waistcoat  with  a  piece  of  crimson ;  and  another  who  put  a  red  collar  on  a 
blue  coat.  Dalton  could  see  in  the  solar  spectrum  only  two  colors,  blue  and 
yellow,  and  having  once  dropped  a  piece  of  red  sealing-wax  in  the  grass,  he 
could  not  distinguish  it. 

t  Moisten  a  swab  with  alcohol  saturated  with  common  salt.  On  ignit- 
ing this  in  a  dark  room,  every  object  will  take  on  a  curious  ghastly  yellow 
hue  from  the  burning  sodium.  The  gay  colors  of  flowers  will  instantly  be 
quenched. 


COMPOSITION     OF     LIGHT. 


217 


FIG.  163. 


Complementary  Colors. 


plementary  to  each  other.  Thus,  if  we  sift  the  red 
rays  out  of  a  beam  of  light  and  bring  the  remainder 
to  a  focus,  a  bluish-green  image 
will  be  formed.*  In  Fig.  163 
the  colors  opposite  each  other 
are  complementary.  Place  a  red 
and  a  blue  ribbon  side  by  side. 
The  former  will  take  on  a  yel- 
lowish and  the  latter  a  greenish 
tint.  Lay  a  piece  of  tissue  pa- 
per upon  black  letters  printed 
on  brightly  colored  paper.  The 
dark  letters  will  appear  of  a  color  complementary  to 
that  of  the  background.! 

9.  The  Rainbow  is  formed  by  the  refraction  and 
reflection  of  the  sunbeam  in  drops  of  falling  water. 
The  white  light  is  thus  decomposed  into  its  simple 
colors.  The  inner  arch  is  termed  the  primary  bow ; 
the  outer  or  fainter  arch,  the  secondary. 

PRIMARY  Bow.— A  ray  of  light,  S",  enters,  and  is 
bent  downward  at  the  top  of  a  falling  drop,  passes 
to  the  opposite  side,  is  there  reflected,  then  passing 
out  of  the  lower  side,  is  bent  upward.  By  the  refrac- 

*  Certain  substances  are  able  to  split  a  ray  of  light  into  two  colors, 
and  are  said  to  be  dichroic.  Gold-leaf  reflects  the  yellow,  transmits  the 
green,  and  absorbs  the  rest. 

t  A  color  is  heightened  when  placed  near  its  complement.  A  red  apple 
is  the  brighter  for  the  contrast  of  the  green  leaf.— Observe  a  white  cloud 
through  a  bit  of  red  glass  with  one  eye  and  through  green  glass  with  the 
other  eye.  After  some  moments,  transfer  both  eyes  to  the  red  glass,  open- 
ing and  closing  them  alternately.  The  strengthening  of  the  red  color  in  the 
eye,  fatigued  by  its  complementary  green,  is  very  striking.— In  examining 
ribbons  of  the  same  color,  the  eye  becomes  wearied  and  unable  to  detect 
the  shade,  because  of  the  mingling  of  the  complementary  hue. 


218  OPTICS. 

tion  the  ray  of  white  light  is  decomposed,  so  that 
when  it  emerges  it  is  spread  out  fan-like,  as  in  the 
solar  spectrum.  Suppose  that  the  eye  of  a  spectator 
is  in  a  proper  position  to  receive  the  red  ray,  he  can 
not  receive  any  other  color  from  the  same  drop,  be- 
cause the  red  is  bent  upward  the  least,  and  all  the 
others  will  pass  directly  over  his  head.  He  sees  the 
violet  in  a  drop  below.  Intermediate  drops  furnish 
the  other  colors  of  the  spectrum. 


FIG.  164. 


The  Rainbow. 

SECOISTDARY  Bow. — A  ray  of  light,  S,  strikes  the 
bottom  of  a  drop,  v,  is  refracted  upward,  passes  to 
the  opposite  side,  where  it  is  twice  reflected,  and 
thence  passes  out  at  the  upper  side  of  the  drop. 
The  violet  ray  being  most  refracted,  is  bent  down  to 
the  eye  of  the  spectator.  Another  drop,  r,  refracting 
another  ray  of  light,  is  in  the  right  position  to  send 
the  red  ray  to  the  eye. 

WHY  THE  Bow  is  CIRCULAR. — When  the  red  ray  of 


COMPOSITION     OF     LIGHT.  219 

the  primary  bow  leaves  the  drop,  it  forms  an  angle 
with  the  sun's  ray,  S"r,  of  about  42°,  and  the  violet 
forms  with  it  one  of  40°.  These  angles  are  constant. 
Let  ab  be  a  straight  line  drawn  from  the  sun  through 
the  observer's  eye.  If  produced,  it  would  pass  through 
the  center  of  the  circle  of  which  the  rainbow  is  an 
arc.  This  line  is  termed  the  visual  axis.  It  is  paral- 
lel to  the  rays  of  the  sun  ;  and  when  it  is  also  parallel 
to  the  horizon,  the  rainbow  is  a  semicircle.  Suppose 
the  line  EV  in  the  primary  bow  to  be  revolved 
around  Eb,  keeping  the  angle  ~bE V  unchanged ;  the 
point  V  would  describe  an  arc  of  a  circle  on  the 
sky,  and  every  drop  over  which  it  passed  would  be 
at  the  proper  angle  to  send  a  violet  ray  to  the  eye 
at  E.  Imagine  the  same  with  the  drop  r.  We  can 
thus  see  (a)  the  bow  must  be  circular ;  (&)  when  the 
sun  is  high  in  the  heavens,  the  whole  bow  sinks 
below  the  horizon;  (c)  the  lower  the  sun  the  larger 
is  the  visible  circumference ;  and  (d)  on  lofty  mount- 
ains a  perfect  circle  may  sometimes  be  seen.* 

1C.  Chromatic  Aberration  of  Lenses.  —  Since  in 
passing  through  any  medium,  such  as  a  lens,  the 
violet  rays  are  bent  farther  away  from  their  first 
direction  than  the  red  rays,  they  will  be  brought  to 
a  focus  nearer  than  that  of  the  red  rays.  An  image 
on  a  screen  produced  with  a  single  lens  is  therefore 
fringed  with  a  reddish  or  a  bluish  fringe  according 

*  Halos,  coronas,  sun-dogs,  circles  about  the  moon,  and  the  tinting  at 
sunrise  and  sunset,  are  produced  by  the  refraction  and  reflection  of  the 
sun's  rays  by  the  clouds.  The  phenomenon  known  as  the  "  sun's  drawing 
water,"  consists  of  the  long  shadows  of  broken  clouds.  Twilight  and  kin- 
dred topics  are  treated  in  Astronomy. 


220  OPTICS. 

to  the  position  of  the  screen.  By  combining  two 
lenses  properly,  one  made  of  crown  glass  and  the 
other  of  flint  glass,  it  is  possible  to  correct  much  of 
this  coloring,  which  is  called  chromatic  aberration, 
and  at  the  same  time  much  of  the  spherical  aberra- 
tion.* 

11.  Interference  of  Light   (Newton's  Rings).  —  Let 
the  convex  side   of  a  plano-convex  lens  be  pressed 
down  upon  a  plane  of  glass.    The  two  surfaces  will 
FIG  165  apparently   touch   at   the    center. 

If    different   circles    be    described 
Newton's      around  this  point,  at  all  parts  of 


Rings*  each    circle  the   surfaces  will  be 

the  same  distance  apart,  and  the  larger  the  circle  the 
greater  the  distance.  Now  let  a  beam  of  red  light  fall 
upon  the  flat  surface.  A  black  spot  is  seen  at  the  cen- 
ter ;  around  this  a  circle  of  red  light,  then  a  dark  ring, 
then  another  circle  of  red  light,  and  so  alternating 
to  the  circumference.  The  distances  between  the 
surfaces  of  the  glass,  where  the  successive  dark  rings 
appear,  are  proportional  to  the  numbers  0,  2,  4  .  .  .  .  , 
and  the  bright  circles  to  1,  3,  5  .....  This  fact 
suggests  the  cause.  There  are  two  sets  of  waves, 
one  reflected  from  the  upper  surface  of  the  plane 
glass,  and  the  other  from  the  lower  surface  of  the 

*  The  crown-glass  lens  must  be  bi-convex  and  the  flint-glass  lens  plano- 
concave or  meniscus.  Flint  glass  gives  a  spectrum  nearly  twice  as  long  as 
crown  glass.  The  two  lenses  oppose  each  other  in  their  action  on  light. 
They  may  be  so  adjusted  that  each  tends  almost  completely  to  reverse  the 
spectrum  that  the  other  would  produce,  and  yet  the  excess  of  deviation 
produced  by  the  crown  glass  may  still  be  enough  to  bring  the  rays  of  this 
nearly  white  light  to  a  focus. 


COMPOSITION     OF     LIGHT.  221 

convex  glass.  These  alternately  interfere,  producing 
darkness,  and  combine,  making  an  intenser  color.* 
To  determine  the  length  of  a  wave  of  red  light,  we 
have  only  to  measure  the  distance  between  the  two 
glasses  at  the  first  ring. 

When  beams  of  light  of  the  various  colors  are 
used  corresponding  circles  are  obtained,  having  dif- 
ferent diameters ;  red  light  gives  the  largest,  and 
violet  the  smallest.  We  hence  conclude  that  red 
waves  are  the  longest,  and  violet  the  shortest.  The 
minuteness  of  these  waves  passes  comprehension. 
About  40,000  red  waves,  or  60,000  violet  ones,  are 
comprised  within  a  single  inch.  Knowing  the  veloc- 
ity of  light,  we  can  calculate  how  many  of  these 
tiny  waves  reach  our  eyes  each  second.  When  we 
look  at  a  violet  object,  757  million  million  of  ether- 
waves  break  on  the  retina  every  moment! 

12.  Polarization  of  Light. — (1.)  DEFINITION. — If  we 
could  look  at  the  end  of  a  ray  of  light  coming  to- 

*  The  play  of  colors  in  mother-of-pearl  is  due  to  the  interference  of 
light  in  its  thin  overlapping  plates. — In  a  similar  manner  the  plumage  of 
certain  birds  reflects  changeable  hues.— A  metallic  surface  ruled  with  fine 
parallel  lines  not  more  than  5-^5  of  an  inch  apart,  gleams  with  brilliant 
colors. — Thin  cracks  in  plates  of  glass  or  quartz,  mica  when  two  layers  are 
slightly  separated,  even  the  scum  floating  in  stagnant  water,  breaks  up  the 
white  light  of  the  sunbeam  and  reflects  the  varying  tints  of  the  rainbow.— 
The  rich  coloring  of  a  soap-bubble  is  caused  by  the  interference  of  the  rays 
reflected  from  the  upper  and  lower  surfaces  of  the  bubble.—  DIFFRACTION  is 
interference  produced  by  a  beam  of  light  passing  along  the  edge  of  an 
opaque  body  or  through  a  small  opening,  or  reflected  by  a  surface  ruled 
with  fine  lines.— Examples ;  Place  the  blades  of  two  knives  closely  together 
and  hold  them  up  to  the  sky ;  waving  lines  of  interference  will  shade  the 
open  space.— Look  at  the  sky  through  the  meshes  of  a  veil,  or  at  a  lamp- 
light through  a  bird-feather  or  a  fine  slit  in  a  card,  and  delicate  colors  like 
those  of  the  prism  will  appear. 


222 


OPTICS. 


FIG.  166. 


FIG.  167. 


FIG.  168. 


ward  us,  as  we  can  at  the  end  of  a  rod,  we  should 
see  the  molecules  of  ether  vibrating  across  the  direc- 
tion of  the  ray  in  all  possible  planes,  as 
shown  in  Fig.  166.  There  are  certain 
conditions  under  which  reflected  or  re- 
fracted light  may  be  made  to  vibrate  in 
but  a  single  plane.  It  is  then  called 
polarized  light. 

The  crystal  tourmaline  has  this  power  upon  trans- 
mitted light.  If  two  thin  plates  of  this  be  cut  parallel 
to  the  axis  of  the  crystal  and  light  be  passed  perpen- 
dicularly through  them,  when  one  is  placed  parallel 
to  the  other,  as  in  Fig.  167,  some  of  it  will  be  ab- 
sorbed, but  what  passes  through 
vibrates  only  in  a  plane  the 
same  as  that  of  the  axis.  This 
is  proved  by  crossing  them,  as 
in  Fig.  168;  at  once  the  light 
is  quenched.  What  passed  Tourmalines 
through  the  first  plate  had  been  Paralleh 
polarized,  and  was  stopped  by  the  second  plate  when 
crossed.  If  they  be  placed  with  axes  oblique  to  each 
other,  part  of  the  polarized  light  is  transmitted  and 
part  quenched. 

(2.)  DOUBLE  REFRACTION. — In  tourmaline  and  many 
other  crystals  the  ether  is  unequally  elastic  in  two 
directions  at  right  angles  to  each  other.  The  light 
is  hence  divided  into  two  parts  which  pass  through 
with  unequal  velocities.  If  transmitted  across  the 
axis  of  the  crystal,  these  parts  are  separated  so  that 
two  beams  become  perceptible.  Iceland  spar  shows 


COMPOSITION     OF     LIGHT.  223 

this  remarkably  well.  An  object  viewed  through  it 
appears  double.  If  the  crystal  be  placed  over  a  dot 
and  turned  around,  two  dots  will  be  seen;  one  ap- 
pears a  little  nearer  than  the  other  and  revolves 
around  it,  or  a  word  will  appear  double  if  viewed  in 
like  manner.  (Fig.  169.)  FIG.  109. 

If  now  a  plate  of  tour- 
maline be  put  between 
the  eye  and  the  rotating 
crystal  of  spar,  the  dots 
will  alternately  disappear. 
This  shows  that  the  two 
beams  were  polarized  at  Double  Refraction- 

right  angles  to  each  other.  One  of  them  is  called 
the  ordinary  and  the  other  the  extraordinary  ray. 
Tourmaline  is  a  doubly  refracting  crystal  in  which 
the  ordinary  ray  is  absorbed  unless  the  plate  be  ex- 
ceedingly thin. 

(3.)  POLARIZATION  BY  REFLECTION. — When  light  falls 
upon  a  surface  of  glass  at  such  an  angle  that  the 
reflected  and  refracted  beams  are  at  right  angles  to 
each  other,  each  of  these  is  polarized,  just  as  in  pass- 
ing through  a  doubly-refracting  crystal.  This  special 
polarizing  angle  of  incidence  for  glass  is  about  56°. 
Many  other  substances  polarize  the  light  reflected  at 
the  proper  angle  from  them.* 

(4.)  THE  POLARISCOPE. — The  best  polarizer  is  a 
crystal  of  Iceland  spar  specially  arranged  so  as  to 

*  If  a  tourmaline  is  rotated  before  the  eye  while  looking  obliquely  at 
the  surface  of  a  varnished  table,  or  leather-seated  chair,  the  reflected  light 
will  be  found  to  be  polarized. 


224  OPTICS. 

transmit  the  extraordinary  ray  and  quench  the  ordi- 
nary ray.  It  is  called  a  Nicol's  prism.  Whatever  is 
used  for  examining  the  light  after  it  has  been  polar- 
ized is  called  an  analyzer.  The  Mcol's  prism  makes 
the  best  analyzer  also.  An  instrument  that  includes 
both  polarizer  and  analyzer  is  called  a  polariscope. 
A  glass  plate  fixed  at  the  proper  angle  makes  an 
excellent  polarizer,  and  a  small  Nicol's  prism,  or  piece 
of  tourmaline,  for  analyzer  is  enough  for  many  beau- 
tiful experiments.  Exquisite  displays  of  complement- 
ary colors,  due  to  interference  of  polarized  beams  in 
transmission,  nfay  be  seen  by  examining  thin  pieces 
of  crystallized  gypsum,  mica,  horn,  strained  glass, 
etc.,  between  polarizer  and  analyzer.*  Polarized  light 
affords  a  delicate  means  of  examining  the  molecular 
structure  of  a  body. 

*  A  simple  polariscope  is  shown  in  Fig.  170.  Upon  a  wooden  frame  a 
plate  of  glass,  P,  blackened  on  the  under  side,  is  fixed  so  that  light  falling 
on  it  at  the  polarizing  angle  shall  be  reflected  through  the  tube  W.  This 
contains  a  small  Nicol's  prism,  n,  for  analyzer,  and  a  lens,  /,  through  which 
an  object,  «,  may  be  examined  with  polarized  light.  The  student  who 


FIG.  170. 


Polariscope. 

makes  one  will  find  it  a  source  of  fascination  and  continued  delight.  He 
will  find  useful  information  very  clearly  expressed  in  the  "  Scientific  Amer- 
ican Supplement"  for  Nov.  20,  1886,  p.  9072;  also,  a  full  and  admirable 
explanation  of  these  beautiful  experiments  in  "Light,"  by  Lewis  Wright, 
published  by  Macmillan  &  Co.,  London. 


OPTICAL    INSTRUMENTS. 


225 


V.    OPTICAL   INSTRUMENTS. 

1.  Microscopes  (to  see  small  things)  are  of  two 
kinds,  simple  and  compound.  The  former  consists  of 
one  or  more  convex  lenses  through  which  the  object 

FIG.  171. 


The  Microscope. 

is  seen  directly;  the  latter  contains  a  simple  magni- 
fier for  viewing  the  image  of  an  object  produced  by 
a  second  lens.  Fig.  171  represents  a  compound  mi- 


226  OPTICS. 

croscope.  At  M  is  a  mirror  which  reflects  the  rays 
of  light  through  the  object  a.  The  object-lens  (ob- 
jective), o,  forms,  in  the  tube  above,  a  magnified,  in- 
verted image  of  the  object.  The  eye-lens,  0  (ocular), 
magnifies  this  image.  The  magnifying  power  of  the 
instrument  is  nearly  equal  to  the  product  of  that  of 
the  two  lenses.  If  a  microscope  increases  the  appar- 
ent diameter  of  an  object  100  times,  it  is  said  to 
have  a  power  of  100  diameters,  the  surface  being 
magnified  1002  =  10,000  times.  The  eye-piece  may 
be  only  a  single  lens,  and  is  really  a  simple  micro- 
scope. The  object-lens  often  consists  of  several  lenses, 
and  each  one  of  a  combination  of  convex  crown  glass 
and  concave  flint  glass  (p.  219)  to  prevent  aberration. 

2.  Telescopes  (to  see  afar  off)  are  of  two  kinds, 
reflecting  and  refracting.  The  former  contains  a 
large  metallic  mirror  (speculum)  which  reflects  the 
rays  of  light  to  a  focus.  The  observer  stands  at  the 
side  and  examines  the  image  with  an  eye-piece.* 


FIG.  172. 


Formation  of  Image  in  the  Telescope. 

The  Refracting  Telescope  contains  an  object-lens, 
0,  which  forms  an  inverted  image,  ab.  This  is  viewed 
through  the  eye-piece,  0,  which  produces  a  magnified 

*  The  largest  reflecting  telescope  is  that  of  Lord  Eosse  (see  Frontispiece 
to  "  Astronomy ").  Its  speculum  is  6  ft.  in  diameter  and  gathers  about 
120,000  times  as  much  light  as  would  ordinarily  enter  the  eye. 


OPTICAL     INSTRUMENTS. 


227 


image,  cd,  of  the  first  image,  ab.  The  image  cd  is 
as  much  larger  than  ab  as  the  focal  distance  of  the 
object-glass  exceeds  that  of  the  eye-glass.  The  larger 


FIG.  173. 


Cambridge  Equatorial. 

the  object-lens  the  more  light  is  collected  with  which 
to  view  the  image.  The  magnifying  power  is  due 
to  the  eye-piece.*  The  apparent  inversion  of  the  ob- 

*  The  use  of  the  telescope  depends  upon  (1st)  its  light-collecting  and 
(2d)  its  magnifying  power.  Thus  Herschel,  illustrating  the  former  point, 
says  that  once  he  told  the  time  of  night  from  a  clock  on  a  steeple  invisible 
on  account  of  the  darkness.  It  is  noticeable  that  while  in  the  compound 
microscope  the  image  is  as  much  larger  than  the  object  as  the  image  is 
farther  than  the  object  from  the  object-glass,  in  the  telescope  the  image  is 
as  much  smaller  than  the  object  as  it  is  nearer  than  the  object  to  the  ob- 
ject-glass ;  while  in  both  cases  the  image  is  examined  with  a  magnifier.  If 


228 


OPTICS. 


ject  is  of  no  importance  for  astronomical  purposes. 
In  terrestrial  observations  additional  lenses  are  used 
to  erect  the  image. 

3.  The  Opera-glass  contains  an  object-glass,  0, 
and  an  eye-piece,  o.  The  latter  is  a  double-concave 
lens ;  this  increases  the  visual  angle  by  diverging 

FIG.  174. 


Formation  of  Image  in  Opera  glass. 

the  rays  of  light,  which  would  otherwise  come  to  a 
focus  beyond  the  eye -piece.  An  erect  and  magni- 
fied image  is  seen  at  ab. 

4.  The  Projecting  Lantern  consists  of  a  system 
of  lenses  attached  to  a  dark  box,  within  which  is 
a  powerful  source  of  illumination  such  as  the  elec- 
tric light  or  lime  light.  Sometimes  an  oil-lamp  is 
used.  From  the  white-hot  source  the  light  is  con- 
verged by  the  condensing  lens,  C,  Fig.  175,  so  as  to 
send  through  the  projecting  lens,  P,  as  much  of  it  as 
possible.  A  picture  on  glass  is  placed  in  front  of  the 

a  power  of  1,000  "be  used  in  looking  at  the  sun,  we  shall  evidently  see  the 
sun  as  if  it  were  only  93,000  miles  away,  or  less  than  one  half  the  distance 
of  the  moon.  The  same  power  used  upon  the  moon  would  bring  that  body 
apparently  to  within  240  miles  of  us. 

The  National  Observatory  telescope  at  Washington  has  an  object-glass 
26  inches  in  diameter,  and  of  excellent  denning  power.  The  Lick  telescope, 
erected  in  1887  upon  Mt.  Hamilton,  in  California,  has  an  object-glass  just 
one  yard  in  diameter.  Its  light-collecting  power  is  estimated  to  be  about 
30,000  times  that  of  the  unaided  eye. 


OPTICAL     INSTRUMENTS. 


229 


condenser,    and    is    thus    strongly    illuminated.      An 
image  of  it,  greatly  enlarged,  is  formed  by  the  pro- 


FIG.  175. 


Projecting  Lantern  for  the  Lime  Light. 

jecting  lens  and  focalized  on  a  distant  white  screen 
in  a  dark  room.      Dissolving  views  are  produced  by 


Projecting  Lantern  for  the  Oil  Light. 

using  two  lanterns  together.     While   one  view  is  on 
the  screen,  another  is  projected   upon  it.     The   light 


230 


OPTICS. 


PIG.  177. 


is  then  cut  off  from  the  first  lantern  so  as  to  leave 
only  the  second  view.  Fig.  176  is  an  outline  of  one 
form  of  oil-lantern.  The  reflector,  M,  helps  to  illu- 
minate the  transparent  picture,  ab,  in  front  of  the 
condenser. 

5.   The  Camera,  used  by  photographers,  contains 
a  double-convex  lens,  L,  which  throws  an  inverted 

image  of  the  ob- 
ject upon  a  re- 
movable ground- 
glass  screen,  S. 
When  the  focus 
has  been  obtain- 
ed, the  screen  is 
removed  and  a 
slide,  containing 
a  sensitive  film, 
is  inserted  in  its 
place.  ("  Chemis- 
try," p  171.) 


Photographer's  Camera. 


6.  The  Eye  is 

a  unique  optical 
instrument  resembling  a  camera.  The  outer  mem- 
brane is  termed  the  sclerotic  coat,  S  (Fig.  178).  It 
is  tough,  white,  opaque,  and  firm.  A  little  portion  in 
front,  called  the  cornea,  c,  is  more  convex  and  per- 
fectly transparent.  The  middle  or  choroid  coat,  (7,  is 
soft  and  delicate,  like  velvet.  It  lines  the  inner  part 
of  the  eye  and  is  covered  with  a  black  pigment.  Over 
it  the  optic  nerve,  which  enters  at  the  rear,  expands 


OPTICAL    INSTKUMENTS.  231 

in  a  net-work  of  delicate  fibers  termed  the  retina, 
the  seat  of  vision.  Back  of  the  cornea  is  a  colored 
curtain,  hi,  the  iris  (rainbow),  in  which  is  a  round 
hole  called  the  pupil.  The  crystalline  lens,  o,  is  a 
double-convex  lens,  composed  of  concentric  layers 
somewhat  like  an  onion,  weighing  about  four  grains 
and  transparent  as  glass.  Between  the  cornea  and 
the  crystalline  lens  is  a  limpid  fluid  termed  the 
aqueous  humor  ;  while  the  vitreous  humor,  a  trans- 
parent, jelly-like  liquid,  fills  the  space  back  of  the 
crystalline  lens. 


Pia.  178. 


Vertical  Section  of  the  Eye. 

Let  AB  represent  an  object  in  front  of  the  eye. 
Rays  of  light  are  first  refracted  by  the  cornea  and 
aqueous  humor,  next  by  the  crystalline  lens,  and  last 
by  the  vitreous  humor,  forming  on  the  retina  an 
image,  ab*  which  is  real,  inverted,  and  smaller  than 
the  object.  To  render  vision  distinct,  the  rays  must 

*  The  diameter  of  the  eye  is  less  than  an  inch ;  yet,  as  we  look  over  an 
extended  landscape,  every  feature,  with  all  its  variety  of  shade  and  color, 
is  repeated  in  miniature  on  the  retina.  Millions  upon  millions  of  ether 
waves,  converging  from  every  direction,  break  on  that  tiny  beach,  while  we, 
oblivious  to  the  marvelous  nature  of  the  act,  think  only  of  the  beauty  of 
the  revelation.  Yet  in  it  the  physicist  sees  a  new  illustration  of  the  sim- 
plicity and  perfection  of  the  laws  and  methods  of  the  Divine  Workman, 
and  a  continued  reminder  of  His  forethought  and  skill. 


232  OPTICS. 

be  accurately  focused  on  the  retina.  If  we  gaze 
steadily  at  an  object  near  by,  and  at  the  same  time 
regard  a  distant  object  in  the  same  direction,  we  find 
our  vision  of  this  blurred.  If  now  we  gaze  at  the 
more  distant  object,  our  vision  of  the  nearer  one  be- 
comes blurred.  The  eye  thus  has  the  power  of  adapt- 
ing itself  to  the  varying  distances  of  objects.  This 
is  done  by  a  change  in  the  convexity  of  the  front 
surface  of  the  crystalline  lens  under  the  action  of 
the  ciliary  muscle  which  surrounds  it  at  its  edge. 
When  clear  vision  can  not  be  had  of  distant  objects, 
the  person  is  near-sighted.  When  the  ciliary  muscle 
is  strained  to  produce  clear  vision  of  objects  less 
than  ten  or  twelve  inches  distant,  the  person,  if 
young,  is  over-sighted.  In  the  first  case,  the  distance 
of  the  retina  from  the  crystalline  lens  is  too  great 
to  permit  of  distinct  f ocalization ;  in  the  second  case, 
this  distance  is  too  small.  The  remedy  for  near- 
sightedness  is  to  wear  concave  glasses,  selected  by  a 
competent  oculist.  Rays  from  distant  objects  are 
thus  made  to  diverge  before  entering  the  eye,  as  if 
they  had  come  from  very  near  objects.  For  over- 
sightedness,  the  remedy  is  properly  selected  convex 
glasses.*  As  old  age  approaches,  the  crystalline  lens 


*  There  are  other  defects  for  which  the  aid  of  the  oculist  should  be 
sought.  If  glasses  are  needed,  they  should  never  be  selected  except  after 
examination  by  a  thoroughly  competent  person.  If  not  properly  adapted, 
they  may  do  much  more  harm  than  good.  No  eye  is  optically  perfect,  and 
but  few  are  free  from  defects  that  may  be  detected  on  examination.  That 
glasses  are  more  used  now  than  during  the  previous  generations  is  due  not 
so  much  to  increase  of  habits  injurious  to  vision  as  to  the  better  knowledge 
of  the  eye  and  the  better  opportunities  for  every  person  to  find  out  his  own 
defects. 


OPTICAL    INSTRUMENTS.  233 

becomes  less  elastic  so  that  tbe  eye  loses  the  power 
of  accommodation  to  near  objects.  Convex  glasses 
become  necessary  for  reading,  while  the  vision  of 
distant  objects  may  remain  perfect. 

The  retina  retains  an  impression  for  a  brief  time 
after  the  object  has  been  removed,  usually  a  fraction 
of  a  second,  which  varies  according  to  the  bright- 
ness.* This  explains  why  a  lighted  coal,  rapidly 
moved  in  the  dark,  appears  as  a  line  of  light.  Many 
of  the  most  brilliant  effects  from  fire-works  depend 
on  this  property  of  the  retina.  The  Zoetrope  is  an 
instrument  by  which  a  succession  of  pictures  of  the 
same  object  in  different  phases  of  motion  are  made 
to  pass  rapidly  before  the  eye.  The  persistence  of 
the  successive  sensations  causes  an  apparent  blending, 
so  that  the  illusion  is  that  of  an  object  actually  in 
motion. 

7.  Binocular  Vision. — In  looking  with  both  eyes 
at  an  object  that  is  not  very  distant,  we  obtain  a 
much  better  idea  of  its  position  and  form,  or  its 
"  depth  in  space,"  than  when  a  single  eye  is  employed. 
The  two  retinal  images  differ  slightly  because  the 
two  eyes  are  different  in  direction  from  the  object, 
and  hence  to  a  slight  extent  we  see  around  the  ob- 
ject on  two  sides.  The  illusion  of  depth  in  space  is 
well  brought  out  by  means  of  Fig.  179.  The  tunnel, 
A,  appears  as  if  viewed  by  the  left  eye  alone  ;  B,  as  if 

*  When  one  is  riding  slowly  on  the  cars  and  looking  at  the  landscape 
between  the  upright  fence-boards,  he  catches  only  glimpses  of  the  view ; 
but  when  mooing  rapidly,  these  snatches  will  combine  to  form  a  perfect  landscape, 
which  has,  however,  a  grayish  tint,  owing  to  the  decreased  amount  of  light 
reflected  to  the  eye. 


234  OPTICS. 

by  the  right  eye  alone.  Bring  the  page  close  up  to 
the  face,  so  that  one  picture  is  immediately  in  front 
of  each  eye.  The  two  images  at  once  seem  combined 
into  a  single  blurred  image.  Now  withdraw  the  page 
a  few  inches;*  the  haziness  gives  place  to  distinct- 

FIG.  179. 


A  B 

A  Stereograph  which  may  be  Viewed  without  a  Stereoscope. 

ness  and  the  tunnel  appears  startlingly  deep,  as  if  it 
were  a  hole  through  the  book.  While  still  gazing 
into  its  depths  two  more  tunnels  may  be  indirectly 
seen,  one  on  each  side  of  it ;  but  the  illusion  of  depth 
in  them  is  far  less  clear.  Each  is  seen  by  but  a  sin- 
gle eye,  while  the  middle  one  is  a  binocular  perception. 
The  Stereoscope  is  an  instrument  intended  to  aid 
in  attaining  binocular  vision  of  a  pair  of  properly 
prepared  pictures,  which  together  compose  the  stereo- 
graph. With  a  little  practice  like  that  just  described, 
any  one  may  become  independent  of  the  stereoscope. 

*  In  performing  this  experiment,  it  is  very  important  to  avoid  crossing 
the  eyes.  Perfect  relaxation  of  the  muscles  of  the  eyeballs  will  make  it 
very  easy.  Imagine  yourself  to  be  looking  thr&ugk  the  page  at  the  opening 
of  a  distant  tunnel,  and  keep  the  muscles  relaxed. 

For  further  discussion  of  the  Stereoscope,  consult  an  article  on  this 
subject  in  the  "Popular  Science  Monthly"  for  May  and  June,  1882. 


PRACTICAL     QUESTIONS.  235 


PRACTICAL     QUESTIONS. 

1.  Why  is  the  secondary  bow  fainter  than  the  primary?    "Why  are  the 
colors  reversed? 

2.  Why  can  we  not  see  around  the  corner  of  a  house,  or  through  a  bent 
tube? 

3.  What  color  would  a  painter  use  if  he  wished  to  represent  an  open- 
ing into  a  dark  cellar? 

4.  Is  white  a  color?    Is  black? 

5.  By  holding  an  object  nearer  a  light,  will  it  increase  or  diminish  the 
size  of  the  shadow? 

6.  What  must  be  the  size  of  a  glass  in  order  to  reflect  a  full-length 
image  of  a  person  ?    Ans.  Half  the  person's  height. 

7.  Where  should  we  look  for  a  rainbow  in  the  morning? 

8.  Can  two  spectators  see  the  same  bow? 

9.  Why,  when  the  drops  of  water  are  falling  through  the  air,  does  the 
rainbow  appear  stationary? 

10.  Why  can  a  cat  see  in  the  night  better  than  a  human  being? 

11.  Why  can  not  an  owl  see  distinctly  in  daylight? 

12.  Why  are  we  blinded  when  we  pass  quickly  from  a  dark  into  a 
lighted  room? 

13.  If  the  light  of  the  sun  upon  a  distant  planet  is  T£o  of  that  which 
we  receive,  how  does  its  distance  from  the  sun  compare  with  ours? 

14.  If,  when  I  sit  six  feet  from  a  candle,  I  receive  a  certain  amount  of 
light,  how  much  shall  I  diminish  it  if  I  move  back  six  feet  farther? 

15.  Why  do  drops  of  rain,  in  falling,  appear  like  liquid  threads? 

16.  Why  does  a  towel  turn  darker  when  wet? 

17.  Does  color  exist  in  the  object,  or  in  the  mind  of  the  observer? 

18.  Why  is  lather  opaque,  while  air  and  a  solution  of  soap  are  each 
transparent  ? 

19.  Why  does  it  whiten  molasses  candy  to  "  pull  it "  ? 

20.  Why  does  plastering  become  lighter  in  color  as  it  dries? 

21.  Why  does  the  photographer  use  a  lamp  with  a  chimney  of  red 
glass  in  the  "  dark  room  "  ? 

22.  Is  the  common  division  of  colors  into '"cold"  and  "warm"  veri- 
fied in  philosophy? 

23.  Why  is  the  image  on  the  camera,  Mg.  177,  inverted? 

24.  Wliy  is  the  second  image  seen  in  a  mirror,  Pig.  140,  brighter  than 
thtf  first? 

25.  Why  does  a  blow  on  the  head  make  one  "see  stars"?     Ans.  The 
blow  excites  the  optic  nerve,  and  so  produces  the  sensation  of  light. 

26.  What  is  the  principle  of  the  kaleidoscope  ?    (If  you  can  not  discover 
this,  consult  Deschanel's  "  Natural  Philosophy,"  pp.  886-891.) 

27.  Which  can  be  seen  at  the  greater  distance— gray  or  yellow? 

28.  When  a  star  is  near  the  horizon,  does  it  seem  higher  or  lower  than 
its  true  place? 


236  OPTICS. 

29.  Why  can  we  not  see  a  rainbow  at  midday? 

30.  What  conclusion  do  we  draw  from  the  fact  that  moonlight  shows 
the  same  dark  lines  in  the  spectrum  as  sunlight? 

31.  Why  does  the   bottom  of  a  boat  seen  under  clear  water  appear 
natter  than  it  really  is? 

32.  Of  what  shape  does  a  round  body  appear  in  water? 

33.  Why  is  rough  glass  translucent  while  smooth  glass  is  transparent  ? 

34.  Why  can  a  carpenter,  by  looking  along  the  edge  of  a  board,  tell 
whether  it  is  straight  ? 

35.  Why  can  we  not  see  out  of  the  window  after  we  have  lighted  the 
lamp  in  the  evening? 

36.  Why  does  a  ground-glass  globe  soften  the  light? 

37.  Why  can  we  not  see  through  ground-glass  or  painted  windows? 

38.  Why  does  the  moon's  surface  appear  flat? 

39.  Why  can  we  see  farther  with  a  telescope  than  with  the  naked  eye? 

40.  Why  is  not  snow  transparent,  like  ice? 

41.  Are  there  rays  in  the  sunbeam  which  we  can  not  perceive  with  the 
eye? 

42.  Why,  when  we  press  the  finger  on  one  eyeball,  do  we  see  objects 
double  ? 

43.  Why  does  a  distant  light,  in  the  night,  seem  like  a  star? 

44.  Why  does  a  bright  light,  in  the  night,  seem  so  much  nearer  than 
it  is? 

45.  What  color  predominates  in  artificial  lights?    Ans.  Yellow. 

46.  Why  are  we  not  sensible  of  darkness  when  we  wink? 

47.  Under  what  condition  do  the  eyes  of  a  portrait  seem  to  follow  a 
spectator  to  all  parts  of  a  room? 

48.  Why  do  the  two  parallel  tracks  of  a  railroad  appear  to  approach  in 
the  distance? 

49.  Why  does  a  fog  apparently  magnify  objects? 

50.  If  you  sit  where  you  can  not  see  another  person's  image,  why  can 
not  that  person  see  yours? 

51.  Why  can  we  see  the  multiple  images  in  a  mirror  better  if  we  look 
Into  it  very  obliquely? 

52.  Why  is  an  image  seen  in  water  inverted? 

53.  Why  is  the  sun's  light  fainter  at  sunset  than  at  midday? 

54.  Why  can  we  not  see  the  fence-posts  when  we  are  riding  rapidly? 

55.  Ought  a  red  flower  to  be  placed  in  a  bouquet  close  to  an  orange 
one?    A  pink  or  blue  with  a  violet  one? 

56.  Why  are  the  clouds  white  while  the  clear  sky  is  blue? 

57.  Why  does  skim-mflk  look  blue  and  new  milk  white? 

58.  Why  is  not  the  image  of  the  sun  in  water  at  midday  so  bright 
as  near  sunset? 

59.  Why  is  the  rainbow  always  opposite  the  sun? 

60.  Hold  a  card  with  its  edge  close  in  front  of  your  eye  and  look  at  a 
distant  candle  flame  in  a  dark  room.    You  will  probably  perceive  either  a 
reddish  or  a  bluish  fringe  on  one  side.    Explain. 


SUMMARY.  237 


SUMMARY. 

Light  comes  from  the  sun  and  other  self-luminous  bodies.  It 
is  transmitted  by  means  of  vibrations  in  ether,  in  accordance 
with  the  laws  of  wave-motion.  It  is  radiated  equally  in  all 
directions,  travels  in  straight  lines,  decreases  as  the  square  of  the 
distance  increases,  and  is  propagated  186,000  miles  per  second. 
Light  falling  upon  a  body  may  be  absorbed,  transmitted,  or  re- 
flected. If  the  surface  be  rough,  the  irregularly-reflected  light 
enables  us  to  see  the  body ;  if  it  be  smooth  and  highly  polished, 
the  rays  are  reflected  so  as  to  form  an  image  of  the  original 
object.  Surfaces  producing  such  images  are  termed  mirrors — 
plane,  concave,  or  convex.  The  image  is  seen  in  the  direction 
from  which  the  reflected  ray  enters  the  eye,  and,  in  a  plane 
mirror,  as  far  behind  the  mirror  as  the  object  is  in  front.  Mul- 
tiple images  are  produced  by  repeated  reflections,  as  in  the 
kaleidoscope.  A  concave  mirror,  as  generally  used,  collects  the 
rays,  and  serves  to  produce  either  a  magnified  erect  virtual 
image  or  a  magnified  or  diminished  inverted  real  image  of  an 
object.  A  convex  mirror  scatters  the  rays,  and  diminishes  the 
apparent  size  of  an  object. 

When  a  ray  enters  or  leaves  a  transparent  body  obliquely,  it  is 
refracted  ;  if  passing  into  a  rarer  medium,  it  is  bent  away  from  the 
perpendicular  erected  at  the  point  of  incidence ;  if  into  a  denser 
medium,  it  is  bent  toward  this  perpendicular.  A  lens  is  a 
transparent  body  with  one  or  more  curved  surfaces,  which  are 
usually  spherical,  so  as  to  refract  the  light  either  to  a  focus,  or 
as  if  it  had  come  from  a  focus.  There  are  two  classes — convex 
and  concave.  The  former  lens,  as  generally  used,  tends,  like  a 
concave  mirror,  to  collect  the  rays  of  light ;  the  latter,  like  a 
convex  mirror,  causes  the  rays  of  light  to  diverge.  Mirage  is 
an  optical  delusion  caused  by  refraction  of  light  in  passing 
through  air  composed  of  strata  of  unequal  density.  Owing 
to  the  varying  refrangibility  of  the  different  waves  of  the 
sunbeam,  a  prism  can  disperse  them  into  a  colored  band  called 
the  solar  spectrum.  The  spectrum  shows  white  light  to  consist 
of  many  tints,  and  that  the  solar  energy  may  produce  lumi- 
nous, heating,  or  chemical  effects  according  to  the  nature  of  the 


238  OPTICS. 

body  receiving  it.  By  means  of  the  spectroscope  we  can  ex- 
amine the  spectrum  of  a  flame,  and  find  whether  its  light  is 
due  to  the  incandescence  of  a  gas  or  to  the  glowing  of  solid 
particles  disseminated  through  it.  Each  substance  'in  the  gaseous 
state  gives  a  spectrum  with  its  peculiar  lines  of  color.  A  gas  ab- 
sorbs the  same  rays  that  it  is  capable  of  emitting ;  if.  therefore,  a 
burning  gas  or  vapor  is  interposed  between  the  eye  and  a  glow- 
ing solid,  the  spectrum  of  the  solid  is  interrupted  by  dark  lines 
due  to  absorption  by  the  vapor.  A  delicate  mode  of  analysis  is 
thus  furnished,  whereby  the  elements  even  of  the  distant  stars 
can  be  detected.  The  rainbow  is  formed  by  the  refraction 
and  reflection  of  the  sunbeam  in  rain-drops.  Light,  when 
reflected  by.  or  transmitted  through  bodies,  is  so  modified, 
chiefly  by  absorption,  as  to  produce  the  varied  phenomena  of 
color.  Each  color  has  its  own  wave-length,  which  is  less  than 
-55~Gtt  inch.  Different  systems  of  light-waves,  as  of  sound- 
waves, may  be  combined.  But  if  any  two  coincide  with  simi- 
lar phases  they  will  strengthen  each  other ;  and  if  with  opposite 
phases,  weaken  each  other.  Interference  of  light,  as  thus  pro- 
duced, causes  the  play  of  colors  in  the  soap-bubble,  mother-of- 
pearl,  etc.  Polarized  light  is  that  in  which  the  molecular  vibra- 
tions are  made  in  the  same  plane.  Many  of  the  most  beauti- 
ful color  effects  may  be  produced  by  polarization. 

The  principal  optical  instruments,  including  the  eye,  are 
adapted  to  produce  and  examine  the  image  formed  by  a  lens. 
In  the  projecting  lantern  and  solar  microscope,  the  image  is 
thrown  on  a  screen  in  a  dark  room.  In  the  refracting  telescope 
and  the  microscope,  the  image  is  formed  in  a  tube  by  a  lens  at 
one  end  and  looked  at  from  behind  by  a  lens  at  the  other  end. 
In  the  ey  ,  which  is  a  small  camera-obscura,  the  image  is  formed 
on  the  retina,  whence  the  sensation  is  carried  by  the  optic  nerve 
to  the  brain.  The  retinal  sensation  continues  for  a  short  time 
after  the  impression  is  made.  Advantage  is  taken  of  this  fact 
in  the  use  of  the  zoetrope,  by  which  a  succession  of  images  is 
made  to  appear  in  motion.  Vision  with  two  eyes  is  superior 
to  that  with  a  single  eye,  because  we  are  thus  enabled  to  form 
better  ideas  of  depth  in  space,  and  hence  of  the  distance  and 
form  of  a  body.  The  stereoscope  is  an  instrument  for  studying 
the  peculiarities  of  binocular  vision. 


HISTORICAL     SKETCH.  239 


HISTORICAL     SKETCH. 

THE  ancients  knew  that  light  is  propagated  in  straight  lines. 
They  discovered  the  laws  of  reflection,  and  one  of  the  ancient 
fables  is  that  Archimedes  set  fire  to  the  Roman  ships  off  Syra- 
cuse by  means  of  concave  mirrors.  Euclid  and  Plato,  however, 
thought  that  the  ray  of  light  proceeds  from  the  eye  to  the  ob- 
ject, an  error  that  was  long  uncorrected.  One  thousand  years  did 
not  bring  much  advancement  in  this  department  of  knowledge. 
The  Arabian  philosopher,  Alhazen,  who  lived  in  the  eleventh 
century,  discovered  the  apparent  displacement  of  a  body  seen 
in  water.  The  law  of  intensity  of  light  was  established  by 
Kepler,  and  the  first  researches  on  the  comparison  of  intensity 
from  different  sources  were  made  by  Maurolycus,  Huygens, 
and  Francis  Marie.  About  1608,  the  telescope  was  invented  by 
the  Dutch.*  Jansen,  Metius,  and  Lippersheim  each  claimed  the 
honor,  and  the  legend  is  that  the  discovery  grew  out  of  some 
children  at  play,  accidentally  arranging  two  watch-glasses  so  as 
apparently  to  magnify  an  object.  In  fact,  however,  the  action 
of  the  convex  lens  was  already  known,  the  compound  microscope 
had  been  invented  by  Jansen  twenty  years  previously,  and  the 
simple  microscope  was  known  to  the  ancient  Chaldeans.  In 
1621,  Snell  discovered  the  law  of  refraction.  By  its  aid  Des- 
cartes explained  the  rainbow.  Half  a  century  of  waiting,  and 
Newton  published  his  investigations  in  the  decomposition  of 
light.  He,  however,  believed  in  what  is  known  as  the  "corpus- 
cular theory."  This  holds  that  light  consists  of  minute  particles 
of  matter  radiated  in  straight  lines  from  a-  luminous  object,  the 
ray  being  endued  with  alternate  "fits"  of  easy  reflection  and 
easy  transmission.  In  1676,  Roemer,  by  observing  Jupiter's 
moons  (p.  192),  found  out  the  velocity  of  light,  which  up  to 
that  time  had  been  considered  instantaneous.  In  1665,  Gri- 

*  "In  1609,  the  government  of  Venice  made  a  considerable  present  to 
Signor  Galileo,  of  Florence,  Professor  of  Mathematics  at  Padua,  and  in- 
creased his  annual  stipend  by  100  crowns,  because,  with  diligent  study,  he 
found  out  a  rule  and  measure  by  which  it  is  possible  to  see  places  30  miles 
distant  as  if  they  were  near,  and,  on  the  other  hand,  near  objects  to  appear 
much  larger  than  they  are  before  our  eyes."— From  an  old  paper  in  the  Library 
of  Heidelberg  University. 


240  OPTICS. 

maldi  discovered  the  existence  of  fringes  of  light  and  shade 
when  a  beam  is  received  through  a  narrow  slit.  Huygens  soon 
afterward  advanced  the  undulatory  theory,  which  was  originated 
independently  about  the  same  time  by  Hooke.  This  involved 
them  in  vigorous  disputes  with  Newton,  without  the  definite 
establishment  of  their  theory.  In  1802,  Thomas  Young  revived 
the  undulatory  theory,  accounting  by  it  for  all  the  phenomena 
of  interference  then  known.  In  1817,  Fresnel  extended  the 
researches  of  Young,  and  Newton's  corpuscular  theory  began  to 
fall  into  discredit.  The  elementary  phenomena  of  polarization 
were  discovered  by  Malus  in  1808,  and  this  subject  was  after- 
ward studied  with  great  thoroughness  by  Fresnel,  Arago,  Biot, 
and  Brewster. 


VIII. 

ON  HEAT. 


"  THE  combustion  of  a  single  pound  of  coal,  supposing  it  to  take  place 
in  a  minute,  is  equivalent  to  the  work  of  three  hundred  horses ;  and  the 
force  set  free  in  the  burning  of  300  Ibs.  of  coal  is  equivalent  to  the  work 
of  an  able-bodied  man  for  a  life-time." 


ANALYSIS  OF  HEAT. 


PRODUCTION 
HEAT. 


OF 


II. 


PHYSICAL  EFFECTS 
OF  HEAT. 


III. 


COMMUNICATION  OF 
HEAT. 


THE    STEAM -EN- 
GINE. 


.   V,   METEOROLOGY. 


1.  Definitions. 

2.  Relation  between  the  Forms  of 

Radiant  Energy. 

3.  Theory  of  Heat. 

4.  Sources  of  Heat. 

5.  Mechanical  Equivalent  of  Heat, 

1.  Expansion. 

2.  Temperature. 

3.  The  Heat  Unit. 

4.  Liquefaction. 

5.  Vaporization. 

6.  Evaporation. 

7.  Spheroidal  State. 

8.  Specific  Heat. 

1.  Conduction. 

2.  Convection. 
]  3.  Radiation. 

I  4.  Absorption  and  Reflection. 

{1.  General  Principle. 
2.  The  Q-overnor. 
3.  The  High-pressure  Engine. 

1.  General  Principles. 

2.  Dew. 

3.  Fogs. 

4.  Clouds. 

5.  Rain. 

6.  Winds. 

7.  Ocean  Currents, 

^  8.  Adaptations  of  Water. 


HEAT. 


I.    PRODUCTION   OF  HEAT. 

1.  Definitions. — Radiant  Energy  is  the  name  of 
what    we    receive    from    the    sun,    stars,    and    other 
heated   bodies.     It    may  be  manifested  as  light,   as 
temperature,  as  chemism,  or  in  all  of  these  ways  at 
the  same  time. 

2.  Relation  between  the  Forms  of  Radiant  En- 
ergy.— Thrust  a  cold  iron  into  the  fire.    It  is  at  first 
dark,  but   soon  becomes   luminous,  like  the  glowing 
coals. — Raise    the    temperature  of   a   platinum  wire. 
We   quickly  feel  the  radiation  of  obscure  heat-rays. 
As  the  metal  begins  to  glow,  our  eyes  detect  a  red 
color,    then    orange    combined    with    it,    and    so    on 
through  the  spectrum.      At   last  all  the  colors  are 
emitted,  and  the  metal  is  dazzling  white.    Like  light, 
heat  may  be  reflected,  refracted,  'and  polarized.     It 
radiates  in  straight  lines  in  every  direction,  and  de- 
creases  in  intensity  as    the   square  of  the    distance 
increases.     It  moves  with  the  same  velocity  as  light. 

It  is  believed  that  each  of  the  forms  of  radiant 
energy  is  merely  the  manifestation  of  wave-motion 
at  a  special  rate.*  The  longer  and  slower  waves  of 

*  According  to  Tyndall,  95  per  cent,  of  the  rays  from  a  candle  are 


244  HEAT. 

ether  falling  upon  the  nerves  of  touch  produce  the  sen- 
sation of  heat.  The  more  rapid  affect  the  optic  nerve 
and  produce  the  sensation  of  light.  The  shortest 
are  especially  active  in  producing  chemical  changes. 

3.    Theory  of  Heat. — Heat  is  motion.     The  inole- 

invisible  or  heat-rays.  These  may  be  brought  to  a  focus  and  bodies  fired 
in  the  darkness.— Each  of  the  five  classes  of  nerves  seems  to  be  adapted  to 
transmit  vibrations  of  its  own  kind,  while  it  is  insensible  to  the  others. 
Thus,  if  the  rate  of  oscillation  be  less  than  that  of  red,  or  more  than  that 
of  violet,  the  optic  nerve  is  uninfluenced  by  the  waves.  We  can  not  see 
with  our  fingers,  taste  with  our  ears,  or  hear  with  our  nose.  Yet  these  are 
organs  of  sensation  and  sensitive  to  their  peculiar  impressions.—"  Suppose, 
by  a  wild  stretch  of  imagination,  some  mechanism  that  will  make  a  rod 
turn  round  one  of  its  ends,  quite  slowly  at  first,  but  then  faster  and  faster, 
till  it  will  revolve  any  number  of  times  in  a  second ;  which  is,  of  course, 
perfectly  imaginable,  though  you  could  not  find  such  a  rod  or  put  together 
such  a  mechanism.  Let  the  whirling  go  on  in  a  dark  room,  and  suppose  a 
man  there  knowing  nothing  of  the  rod ;  how  will  he  be  affected  by  it  ?  So 
long  as  it  turns  but  a  few  times  in  the  second,  he  will  not  be  affected  at 
all  unless  he  is  near  enough  to  receive  a  blow  on  the  skin.  But  as  soon  as 
it  begins  to  spin  from  sixteen  to  twenty  times  a  second,  a  deep  growling 
note  will  break  in  upon  him  through  his  ear ;  and  as  the  rate  then  grows 
swifter,  the  tone  will  go  on  becoming  less  and  less  grave,  and  soon  more 
and  more  acute,  till  it  will  reach  a  pitch  of  shrillness  hardly  to  be  borne, 
when  the  speed  has  to  be  counted  by  tens  of  thousands.  At  length,  about 
the  stage  of  forty  thousand  revolutions  a  second,  more  or  less,  -the  shrill- 
ness will  pass  into  stillness ;  silence  will  again  reign  as  at  first,  nor  any 
more  be  broken.  The  rod  might  now  plunge  on  in  mad  fury  for  a  long 
time  without  making  any  difference  to  the  man ;  but  let  it  suddenly  come 
to  whirl  some  million  times  a  second,  and  then  through  intervening  space 
faint  rays  of  heat  will  begin  to  steal  toward  him,  setting  up  a  feeling  of 
warmth  in  his  skin ;  which  again  will  grow  more  and  more  intense,  as 
now  through  tens  and  hundreds  and  thousands  of  millions  the  rate  of 
revolution  is  supposed  to  rise.  Why  not  billions?  The  heat  at  first  will 
be  only  so  much  the  greater.  But,  lo  1  about  the  stage  of  four  hundred 
billions  there  is  more— a  dim  red  light  becomes  visible  in  the  gloom;  and 
now,  while  the  rate  still  mounts  up,  the  heat  in  its  turn  dies  away,  till  it 
vanishes  as  the  sound  vanished ;  but  the  red  light  will  have  passed  for  the 
eye  into  a  yellow,  a  green,  a  blue,  and,  last  of  all,  a  violet.  And  to  the 
violet,  the  revolutions  being  now  about  eight  hundred  billions  a  second, 
there  will  succeed  darkness  —  night,  as  in  the  beginning.  This  darkness 
too,  like  the  stillness,  will  never  more  be  broken.  !Let  the  rod  whirl  on  as 
it  may,  its  doings  can  not  come  within  the  ken  of  that  man's  senses." 


PRODUCTION     OF     HEAT.  245 

cules  of  a  solid  are  in  constant  vibration.  When  we 
increase  the  rapidity  of  this  oscillation,  we  heat  the 
body  ;  when  we  decrease  it,  we  cool  the  body.  The 
vacant  spaces  between  the  molecules  are  filled  with 
ether.  As  the  air  moving  among  the  limbs  of  a 
tree  sets  its  boughs  in  motion,  and  in  turn  may  be 
kept  in  motion  by  the  waving  of  branches,  so  the 
ether  puts  the  molecules  in  vibration,  or  is  thrown 
into  vibration  by  them. — Example :  Insert  one  end  of 
a  poker  in  the  fire.  The  particles  immersed  in  the 
flame  are  made  to  vibrate  intensely ;  the  swinging 
molecules  strike  their  neighbors,  and  so  on,  continu- 
ally, until  the  oscillation  reaches  the  other  end.  If 
we  handle  the  poker,  the  motion  is  imparted  to  the 
delicate  nerves  of  touch ;  they  carry  it  to  the  brain, 
and  pain  is  felt.  In  popular  language,  "  the  iron  is 
hot,"  and  we  are  burned.  If,  without  touching  it, 
we  hold  our  hand  near  the  poker,  the  ether-waves 
set  in  motion  by  the  vibrating  molecules  of  iron 
strike  against  the  hand,  and  produce  a  less  intense 
sensation  of  heat.  In  the  former  case,  the  fierce 
motion  is  imparted  directly ;  in  the  latter,  the  ether 
acts  as  a  carrier  to  bring  it  to  us. 

4.  The  Sources  of  Heat  are  the  sun,  the  stars, 
and  mechanical  and  chemical  energy. 

(1.)  The  molecules  of  the  sun  and  stars  are  in 
rapid  vibration.  These  set  in  motion  waves  of  ether, 
which  are  propagated  across  the  intervening  space, 
and  meeting  the  earth,  give  up  their  motion  to  it. 
(2.)  Friction  and  percussion  produce  heat,  the  mo- 
tion of  a  mass  being  changed  into  motion  among 


246  HEAT. 

molecules.*  (3.)  Chemical  action  is  seen  in  fire.  The 
oxygen  of  the  air  has  an  affinity  for  the  carbon  and 
hydrogen  of  the  fuel.  They  combine,  and  chemical 
energy  is  transformed  into  that  of  sensible  heat. 

5.  Mechanical  Equivalent  of  Heat  (Joule's  Law). 
— In  these  various  changes  of  mechanical  motion 
into  motion  of  molecules  no  energy  is  destroyed, 
though  some  of  it  may  be  so  transformed  as  to  be- 
come incapable  of  being  made  to  do  useful  work. 
If  the  energy  transformed  by  the  fall  of  a  black- 
smith's hammer  on  his  anvil  could  be  gathered  up, 
it  would  be  sufficient  to  lift  the  hammer  to  the  point 
from  which  it  fell.  A  pound-weight  falling  vertically 
772  feet,  will  generate  enough  heat  to  raise  the  tem- 
perature of  1  pound  of  water  through  1°  F.;  con- 
versely, this  amount  of  heat  is  the  equivalent  of  the 
energy  required  to  lift  1  pound  mechanically  to  a 
height  of  772  feet.  This  important  truth  was  first 
demonstrated  by  Mr.  Joule,  of  Manchester,  England, 
and  we  express  it  by  saying  772  foot-pounds  is  the 
mechanical  equivalent  of  heat.  Expressed  in  metric 
measures,  it  is  424  kilogram-meters  for  1°  C. 

*  A  horse  hits  his  shoes  against  a  stone  and  "  strikes  fire  " ;  little  par- 
ticles of  the  metal  being  torn  off  are  heated  by  the  shock,  and  some  of  the 
energy  is  manifested  also  as  light.— A  train  of  cars  is  stopped  by  the  press- 
ure of  the  brakes.  In  a  dark  night,  we  see  the  sparks  flying  from  the 
wheels,  the  motion  of  the  train  being  converted  into  heat.— A  blacksmith 
pounds  a  piece  of  iron  until  it  glows.  His  strokes  set  the  particles  of  metal 
vibrating  rapidly  enough  to  send  ether-waves  of  such  swiftness  as  to  affect 
the  eye  of  the  observer.— As  a  cannon-shot  strikes  an  iron  target,  a  shower 
of  sparks  is  scattered  around.— Were  the  earth  instantly  stopped,  enough 
heat  would  be  produced  to  "  raise  a  lead  ball  the  size  of  our  globe  to  384,000° 
C."  If  it  were  to  fall  to  the  sun  its  impact  would  produce  a  thousand  times 
more  heat  than  its  burning. 


PHYSICAL     EFFECTS     OF     HEAT.  247 

II.    PHYSICAL   EFFECTS  OF   HEAT. 

1.  Expansion. — If  the  molecules  of  a  body  have 
an  increase  of  energy  imparted  to  them  they  swing, 
like  pendulums,  through  wider  arcs.  Each  tends  to 
push  against  its  neighbor,  and  the  mass  as  a  whole 
grows  larger.  Hence  the  general  law,  "Heat  ex- 
pands and  cold  contracts,"  cold  being  merely  a  rela- 
tive term  implying  the  withdrawal  of  energy.  The 
ratio  of  the  increase  of  volume  to  the  original  vol- 
ume for  a  change  of  1°  in  temperature  is  called  the 
Coefficient  of  Expansion.  Generally  this  is  greatest 
for  gases,  less  for  liquids,  and  least  for  solids,  each 
particular  substance  having  its  own  co- efficient. 
The  force  of  expansion  is  for  many  substances 
-irresistible.  A  rise  in  temperature  of  80°  F.  will 
lengthen  a  bar  of  wrought-iron,  10  feet  long,  about 
^  of  an  inch  ;  and  if  its  cross-section  is  one  square 
inch  it  will  push  in  expanding  with  a  force  of  about 
2  5  tons.  When  the  metal  cools  it  will  contract  with 
the  same  force.* 

A  familiar  application  of  expansion  is  in  the  pen- 

*  A  carriage-tire  is  put  on  when  hot,  in  order  that,  when  cooled,  it  may 
bind  the  wheel  together.— Rivets  used  in  fastening  the  plates  of  steam-boilers 
are  inserted  red-hot.—"  The  ponderous  iron  tubes  of  the  Britannia  Bridge 
writhe  and  twist,  like  a  huge  serpent,  under  the  varying  influence  of  the 
solar  heat.  A  span  of  the  tube  is  depressed  only  a  quarter  of  an  inch  by 
the  heaviest  train  of  cars,  while  the  sun  lifts  it  2£  inches."  The  same  may 
be  noticed  on  the  great  Brooklyn  Bridge,  more  than  a  mile  long,  where  an 
allowance  of  nearly  a  yard  has  to  be  made  for  expansion  with  the  change 
of  seasons.— The  Bunker-hill  monument  nods  as  it  follows  the  sun  in  its 
daily  course.— Tumblers  of  thick  glass  break  on  the  sudden  application  of 
heat,  because  the  surface  dilates  before  the  heat  has  time  to  be  conducted 
to  the  interior. 


248 


HEAT. 


PICK  180. 


dulum  of  a  clock,  which  lengthens  in  summer  and 
shortens  in  winter.  A  clock,  therefore,  tends  to  lose 
time  in  summer  and  gain  in  winter.  To 
regulate  it  we  raise  or  lower  the  pendu- 
lum bob. 

The  gridiron  pendulum  consists  of 
brass  and  steel  rods,  so  connected  that 
the  brass,  h,  Jc,  will  lengthen  upward, 
and  the  steel,  a,  &,  c,  d,  downward,  and 
thus  the  center  of  oscillation  remain 
unchanged.  The  mercurial  pendulum 
contains  a  cup  of  mercury  which  ex- 
pands upward,  while  the  pendulum-rod 
expands  downward. 

2.  Temperature. — When  one  body  is 
in  a  condition  to  communicate  heat  to 
another,  the  first  is  said  to  have  a 
higher  temperature  than  the  second,  or 
to  be  warmer.  We  measure  temperature 
usually  by  noting  its  effect  in  producing 
expansion.  Within  narrow  limits  we 
may  form  a  rough  estimate  of  it  by 
the  sensation  of  touch,  but  this  is 

very  unreliable. 

The  thermometer  is  an  instrument  for  measuring 

temperature,  usually  by  the  expansion  of  mercury.* 

*  Take  a  glass  tube  terminating  in  a  bulb,  and  heat  the  bulb  to  expel 
the  air.  Then  plunge  the  stem  in  colored  water.  As  the  bulb  cools,  the 
water  will  rise  and  partly  fill  it.  Heat  the  bulb  again  until  the  steam 
pours  out  of  the  stem.  On  inserting  it  a  second  time,  the  water  will  fill 
the  bulb.  Tn  the  manufacture  of  thermometers,  it  is  customary  to  have  a 
cup  blown  at  the  upper  end  of  the  stem.  This  is  filled  with  mercury,  and 


Gridiron  Pendu- 
lum. 


PHYSICAL     EFFECTS     OF     HEAT. 


249 


FIG.  181. 


To  graduate  it,  according  to  Fahrenheit's  scale 
each  thermometer  is  put  in  melting  ice,  and  the 
point  to  which  the  mercury  sinks  is 
marked  32°,  Freezing-point.*  It  is  then 
placed  in  a  steam-bath,  and  the  point  to 
which  the  mercury  rises  (when  the  baro- 
metric column  stands  at  30  inches)  is 
marked  212°,  Boiling-point.  The  space 
between  these  two  points  is  divided  into 
180  equal  parts.  In  the  Centigrade  scale 
(O.)  the  freezing-point  is  0,  and  the  boil- 
ing-point 100°.  In  Reaumur's  scale  (J2.), 
the  boiling-point  is  80°.f  The  thermom- 
eter does  not  measure  the  quantity  of 
heat,  but  only  its  intensity. 


F.    c.   R. 

Thermometers. 


3.  The  Heat  Unit. — For  measuring 
quantity  of  heat,  the  unit  commonly  em- 
ployed in  England  and  America  is  that 
quantity  which  is  required  to  raise  the  temperature 
of  one  pound  (avoirdupois)  of  water  through  one 
degree  (Fahrenheit)  above  the  freezing-point. 


the  air,  when  expanded,  bubbles  out  through  it,  while  the  metal  trickles 
down  as  the  bulb  cools.  The  mercury  is  th£n  highly  heated,  when  the 
tube  is  melted  off  and  sealed  at  the  end  of  the  column  of  mercury.  The 
metal  contracts  on  cooling,  and  leaves  a  vacuum  above. 

*  The  inventor  placed  zero  32°  below  the  temperature  of  freezing 
water,  because  he  thought  that  to  be  absolute  cold— a  point  now.  estimated 
to  be  about  492°  below  the  freezing-point  on  his  scale. 

t  The  following  formulae  will  be  of  use  in  comparing  the  readings  of 
the  different  scales : 

R.=  |  C.  =  |  (F.  -  32°).     .t  ,    ,...,...    .  (1.) 

C.  =  £  R.  =  |  (F.  -  32°) .......    (2.) 

F.  =  f  C.  +  32°  =  |  R.  +  32° (3.) 

1°  O.  =  1.8°  F.  .    ,    .    .    (4.) 


250  HEAT. 

4.  Liquefaction  or  Fusion. — When  heat  is  commu- 
nicated to  a  solid  body  a  point  is  finally  reached 
when  the  vibratory  swing  of  its  molecules  is  so  great 
that  they  are  driven  apart,  each  toward  the  limit  of 
the  sphere  of  attraction  of  its  neighbor,  so  that  all 
rigidity  is  lost.*  The  molecules  then  move  freely 
among  themselves.  The  energy  that  is  applied  raises 
the  temperature  of  the  body  up  to  a  fixed  point  called 
its  melting  or  fusing  point,  when  liquefaction  begins. 
Additional  energy  then  does  the  work  of  driving  the 
molecules  apart  without  further  rise  of  temperature, 
until  fusion  is  complete  ;  after  which  the  liquid  rises 
still  further  in  temperature.  Energy  that  does  thus 
the  work  of  changing  the  state  of  a  body  without  at 
the  same  time  changing  its  temperature  is  often 
called  latent  heat.\  If  a  pound  of  ice  at  32°  F.  be 
heated,  it  requires  142  heat  units  to  melt  it,  and 
180  more  to  raise  its  temperature  then  up  to  the 
boiling-point. 

Freezing.  —  The  converse  of  fusion  is  freezing. 
Ice  melts  at  32°  F.,  and  in  doing  so.  it  absorbs  en- 
ergy. Water  freezes  at  32°  F.,  and  in  doing  so  it 
gives  out  the  energy  which  had  been  keeping  its 
molecules  apart.  Thawing  is  thus  a  cooling  process 
and  freezing  is  a  warming  process.  Freezing  mixt- 
ures depend  on  this  principle.  In  freezing  ice- 
cream, salt  and  pounded  ice  are  put  around  the 

*  This  is  true  only  of  bodies  which  are  not  broken  into  their  chemical 
constituents  before  the  melting-point  is  reached.  A  large  variety  of  sub- 
stances, such  as  wood*  bone,  flesh,  etc.,  become  chemically  changed  instead 
of  melting. 

t  The  term  latent  heat  is  gradually  going  out  of  use. 


PHYSICAL     EFFECTS     OF     HEAT.  251 

vessel  that  contains  the  cream.  The  strong  attrac- 
tion between  salt  and  water  causes  the  ice  to  melt 
rapidly,  and  the  solid  salt  becomes  liquid  by  solu- 
tion. This  rapid  thawing  involves  much  absorption 
of  energy,  which  comes  from  the  nearest  objects 
whose  temperature  is  higher  than  that  of  the  solu- 
tion. The  cream  thus  loses  energy,  its  temperature 
becoming  reduced  down  to  the  freezing-point.* 

5.  Vaporization.  —  When  heat  is  applied  to  a 
liquid  the  temperature  rises  until  the  boiling-point 
is  reached,  when  it  stops  and  the  liquid  is  changed 
to  vapor  at  that  constant  temperature.  The  vapor 
is  nearly  free  from  solids  dissolved  in  the  liquid. — 
Example:  Pure  or  distilled  water  is  obtained  by 
heating  water  in  a  boiler,  A,  whence  the  steam 
passes  through  the  pipe,  (7,  and  the  worm  within  the 
condenser,  S,  where  it  is  condensed  and  drops  into 
the  vessel,  D.  The  pipe  is  coiled  in  a  spiral  form 
within  the  condenser,  and  is  hence  termed  the 
worm.  The  condenser  is  kept  full  of  cold  water 
from  the  tub  at  the  left.  By  carefully  regulating 
the  temperature,  one  liquid  may  be  separated  from 
another  by  "fractional"  distillation,  advantage  being 
taken  of  the  fact  that  each  liquid  has  its  own  boil- 

*  That  freezing  is  a  warming  process  may  be  conclusively  shown  as  fol- 
lows :  Gently  melt  some  sodium  sulphate  (a  cheap  salt  that  may  be  ob- 
tained from  any  apothecary)  in  a  flask  by  heating  it  over  a  lamp  flame. 
Put  it  aside  to  cool  slowly  in  a  perfectly  quiet  place.  After  cooling  it  re- 
mains liquid,  but  ready  to  freeze  as  soon  as  motion  among  its  molecules  is 
started.  Disturb  it  by  putting  a  thermometer  bulb  into  the  liquid.  At 
once  crystals  are  seen  shooting  out,  and  the  mass  is  soon  frozen  hard.  The 
mercury  in  the  thermometer  meanwhile  rises,  and  the  warming  may  be 
felt  with  the  hand. 


252 


HEAT. 


ing-point,  higher   or   lower  than   that   of  the  liquid 
with  which  it  is  mixed. 

Boiling-point.— When  we  heat  water,  the  bubbles 
which  pass  off  first  are  the  air  dissolved  in  the 
liquid ;  next  bubbles  of  steam  form  on  the  bottom 
and  sides  of  the  vessel,  and,  rising  a  little  distance, 


FIG.  182. 


A  Still. 


are  condensed  by  the  cold  water.  Collapsing,  they 
produce  the  sound  known  as  "simmering."  As  the 
temperature  of  the  water  rises,  they  ascend  higher, 
until  they  burst  at  the  surface,  and  pass  off  into  the 
air.  The  violent  agitation  of  the  water  thus  pro- 
duced is  termed  boiling.*  Some  substances  vaporize 

*  The  temperature  of  water  can  not  be  raised  above  the  boiling-point, 
unless  the  steam  be  confined.  The  extra  energy  is  applied  in  expanding 
the  water  into  stea'm.  This  occupies  1,700  times  the  space,  and  is  of  the 
same  temperature  as  the  water  from  which  it  is  made.  Nearly  1,000°  units 


PHYSICAL     EFFECTS     OF     HEAT.  253 

at  ordinary  temperatures  ;  others  only  at  the  high- 
est ;  while  the  gases  of  the  air  are  but  the  vapor  of 
substances  which  boil  at  exceedingly  low  tempera- 
tures. The  distinction  between  gases  and  vapors  in 
ordinary  language  is  only  relative. 

The  boiling-point  of  water  depends  on  three  cir- 
cumstances: (1.)  Purity  of  the  water.  A  solid  sub- 
stance dissolved  in  water  ordinarily  elevates  the 
boiling-point.  Thus  salt  water  boils  at  a  higher 
temperature  than  pure  water.  The  air  dissolved  in 
water  tends  by  its  elastic  force  to  separate  the  mole- 
cules. If  this  be  removed,  the  boiling-point  may  be 
elevated  to  275°  F.,  when  the  water  will  be  con- 
verted into  steam  with  explosive  violence. 

(2.)  Nature  of  the  vessel.  Water  will  boil  at  a 
lower  temperature  in  iron  than  in  glass.  When  the 
surface  of  the  glass  is  chemically  clean,  the  boiling- 
point  is  still  higher.  This  seems  to  depend  in  some 
degree  on  the  strength  of  the  adhesion  between  the 
water  and  the  containing  vessel. 

(3.)  Pressure  upon  the  surface  raises  the  boiling- 
point.*  Water,  therefore,  boils  at  a  lower  tempera- 
ture on  a  mountain  than  in  a  valley.  The  tempera- 
ture of  boiling  water  at  Quito"  is  194°  F.,  and  on 

of  heat  for  each  pound  of  water  are  expended  in  this  process,  but  are  made 
sensible  again  as  temperature  when  the  steam  is  condensed.  Steam  is  in- 
visible. This  we  can  verify  by  examining  it  where  it  issues  from  the 
spout  of  the  tea-kettle.  It  soon  condenses,  however,  into  minute  globules, 
which  become  visible  in  white  clouds. 

*  Pressure  opposes  the  repellent  heat-force,  and  so  renders  it  easier  for 
cohesion  to  hold  the  particles  together.  In  the  interior  of  the  earth  there 
may  be  masses  of  matter  heated  red  or  white  hot  and  yet  solid,  more 
rigid  even  than  glass,  in  consequence  of  their  melting-point  being  raised 
so  high  by  the  tremendous  pressure  that  they  can  not  liquefy.— TAIT. 


254 


HEAT. 


FIG.  183. 


Mont  Blanc,  183°  F.     The   variation  is  so   uniform 
that  the  height  of  a  place  can  thus  be  a'scertained; 

an  ascent  of  596  feet  pro- 
ducing a  difference  of  1°  F. 
The  influence  of  pressure 
is  well  illustrated  by  the  fol- 
lowing experiment :  Half  fill 
a  strong  glass  flask  with 
water,  and  boil  this  until 
all  the  air  is  expelled  from 
both  the  water  and  the 
space  above  it.  Now  quickly 
apply  a  tight  stopper  and 
invert.  The  pressure  of  the 
steam  will  stop  ebullition. 
A  few  drops  of  cold  water 
will  condense  the  steam,  and 
boiling  will  recommence. 
This  will  soon  be  checked, 
but  can  be  restored  as  before.  The  process  may  be 
repeated  until  the  water  cools  to  the  ordinary  tem- 
perature of  the  air,  and  even  then  the  liquid  inside 
may  be  made  to  boil  by  rubbing  the  outside  of  the 
flask  with  ice.  The  cushion  of  air  which  commonly 
breaks  the  fall  of  water  is  removed,  and  if  the  cork 
be  air-tight,  the  water,  when  cold,  will  strike  against 
the  flask  with  a  sharp,  metallic  sound. 

6.  Evaporation  is  a  slow  formation  of  vapor, 
which  takes  place  at  ordinary  temperatures.  Water 
evaporates  even  at  the  freezing-point.  Clothes  dry 
in  the  open  air  in  the  coldest  weather.  The  wind 


Boiling  Water  by  Condensing  its 
Vapor. 


PHYSICAL     EFFECTS     OF     HEAT.  255 

quickens  the  process,  because  it  drives  away  the 
moist  air  near  the  clothes  and  supplies  dry  air. 
Evaporation  is  also  hastened  by  an  increase  of  sur- 
face and  a  gentle  heat. 

Vacuum  pans  are  employed  in  condensing  milk 
and  in  the  manufacture  of  sugar.  They  are  so  ar- 
ranged that  the  air  above  the  liquid  in  the  vessel 
may  be  exhausted,  and  then  the  evaporation  takes 
place  rapidly,  and  at  so  low  a  temperature  that 
burning  is  avoided. 

The  cooling  effect  of -evaporation  is  due  to  the  ab- 
sorption of  energy  required  to  drive  the  molecules 
apart  beyond  their  spheres  of  mutual  attraction. 
Water  may  be  frozen  under  the  receiver  of  an  air- 
pump  by  placing  a  small  watch-glass  containing  it 
over  a  pan  of  strong  sulphuric  acid,  which  absorbs 
the  vapor  as  fast  as  it  is  formed  in  the  vacuum. 
The  cooling  due  to  rapid  evaporation  of  a  part  is 
sufficient  to  freeze  the  rest.  By  strong  pressure  and 
cooling,  carbonic  acid  is  easily  liquefied.  Allowing  a 
jet  of  this  liquid  to  escape,  the  evaporation  of  a  part 
of  it  causes  the  rest  to  freeze  into  a  snowy  powder 
which  may  be  pressed  into  a  ball.*  Nitrogen,  oxy- 
gen, and  air,  which  is  a  mixture  chiefly  of  these 

*  Mercury  in  contact  with  it  is  quickly  solidified.  On  throwing  the 
frozen  metal  into  a  little  water,  the  mercury  instantly  liquefies,  but  the 
water  turns  to  ice,  the  solid  thus  becoming  a  liquid  and  the  liquid  a  solid 
by  the  exchange  of  heat.  A  cold  knife  cuts  through  the  mass  of  frozen 
mercury  as  a  hot  knife  would  ordinarily  through  butter.  The  author,  on 
one  occasion,  saw  Tyndall,  during  a  course  of  lectures  at  the  Eoyal  Insti- 
tution at  London,  when  freezing  a  ladle  of  mercury  in  a  red-hot  crucible, 
add  some  ether  to  hasten  the  evaporation.  The  liquid  caught  fire,  but  the 
metal  was  drawn  out  from  the  glowing  crucible,  through  the  midst  of  the 
flame,  frozen  into  a  solid  mass. 


256  HEAT. 

two  gases,  have  been  liquefied.      Liquid  air  boils  at 

—  337°  F.  in  a  vacuum.    Nitrogen  has  been  obtained 
in   "snow-like   crystals   of  remarkable   size,"  and  by 
reducing    the    pressure    on    these    a    temperature  of 

—  373°  F.  was   attained, — the  lowest  recorded  up  to 
the  present  date  (1887). 

7.  Spheroidal   State. — If  a  few  drops  of  water  be 
put  in  a  hot,  bright  spoon,  they  will    gather    in  a 
globule,  which  will  dart  to  and  fro  over  the  surface. 
It  rests  on  a  cushion  of  steam,  while  the  currents  of 
air  drive  it  about.      If  the  spoon  cool,  the  water  will 
lose  its  spheroidal  form,  and  coming  into  contact  with 
the  metal,  burst  into  steam  with  a  slight  explosion.* 

8.  Specific    Heat.  —  More    energy   is    required    to 
raise  the  temperature  of  a  pound  of  water  through 
one  degree  than  for  any  other  substance  except  the 
gas  hydrogen.    The  number  of  heat  units  (p.  249,  §  3) 
required  to  raise  one  pound  of  a  given  substance  one 
degree  F.  is  called  its  specific  heat;   thus  for  mer- 
cury   it    is    about    -gV ;    for    iron,  \ ;    for    air,    nearly 
£ ;    for  hydrogen,  3^.      On  this   account  the  ocean 
changes  its  temperature   far   less   quickly  than    the 
land,  and  sea-side  cities  are  subject  to  less  extremes 
of  temperature  than  those  on  the  middle  of  a  conti- 
nent.     On   the   elevated   plateau   region   around   the 
Great   Salt   Lake   the   temperature   during   the   year 
varies  from  115°  F.  to  -  30°  F. 

*  Drops  of  water  spilled  on  a  hot  stove  illustrate*  the  principle.— By 
moistening  the  finger,  we  can  touch  a  hot  flat-iron  with  impunity.  The 
water  assumes  this  state,  and  thus  protects  the  flesh  from  injury.-— 
Furnace-men  can  dip  their  moistened  hands  into  molten  iron. 


COMMUNICATION     OF      HEAT.  257 

III.    COMMUNICATION   OF   HEAT. 

Heat  tends  to  become  diffused  equally  among 
neighboring  bodies.*  There  are  three  modes  of  dis- 
tribution. 

1.  Conduction  is  the  process  of  heating  by  the 
passage  of  heat  from  molecule  to  molecule. — Examr 
pie  :  Hold  one  end  of  a  poker  in  the  fire,  and  the 
other  end  soon  becomes  hot  enough  to  burn  the 
hand.  Of  the  ordinary  metals,  silver  and  copper 
are  the  best  conductors.!  Wood  is  a  poor  con- 
ductor, especially  "  across  the  grain." 

Gases  are  the  poorest  conductors ;  hence  porous 
bodies,  as  wool,  fur,  snow,  charcoal,  etc.,  which  con- 
tain large  quantities  of  air,  are  excellent  non-con- 
ductors. Refrigerators  and  ice-houses  have  double 
walls,  filled  between  with  charcoal,  sawdust,  or  other 
non-conducting  substances.  Air  is  so  poor  a  con- 
ductor that  persons  have  gone  into  ovens  that  were 
hot  enough  to  cook  meat,  which  they  carried  in  and 
laid  on  the  metal  shelves;  yet,  so  long  as  they  did 
not  themselves  touch  any  good  conductor,  they  ex- 
perienced little  inconvenience. 

Liquids    are    also     poor    conductors.  —  Example: 

*  If  we  touch  an  object  colder  than  we  are,  it  abstracts  heat  from  us, 
and  we  say  "  it  feels  cold "  ;  if  a  warmer  body,  it  imparts  heat  to  us,  and 
we  say  "it  feels  warm."  Adjacent  objects  have,  however,  the  same  tem- 
perature, though  flannel  sheets  feel  warm,  and  linen  cold.  These  effects 
depend  upon  the  relative  conducting  power  of  different  substances.  Iron 
feels  colder  than  feathers  because  it  robs  us  faster  of  our  heat. 

t  Place  a  silver,  a  German-silver,  and  an  iron  spoon  in  a  dish  of  hot 
water.  ^Notice  how  much  sooner  the  handle  of  the  silver  spoon  is  heated 
than  the  others. 


258 


HEAT. 


FIG.  184. 


Hold  the  upper  end  of  a  test-tube  of  water  in  the 
flame  of  a  lamp.  The  water  nearest  the  blaze  will 
boil  without  the  heat  being  felt 
by  the  hand. 

2.  Convection  is  the  process 
of  heating  ~by  circulation.  (1.) 
CONVECTION  OF  LIQUIDS. — Place  a 
little  sawdust  in  a  flask  of  water, 
and  apply  heat.  We  shall  soon 
find  that  an  ascending  and  a 
descending  current  are  estab- 
lished. The  water  near  the 
lamp  becoming  heated,  expands 
and  rises.  The  cold  water  above 

Heating  by  Convection.  ginks    to   take   ^  placa 

(2.)  OF  GASES. — By  testing  with  a  lighted  candle, 
we  shall  find  at  the  bottom  of  a  door  opening  into 
cold  air,  a  current  setting  inward,  and  at  the  top, 
one  setting  outward.  The  cold  air  in  a  room  flows 
to  the  stove  along  the  floor,  is  heated,  and  then  rises 
to  the  ceiling.  Heating  by  hot-air  furnaces  depends 
upon  the  principle  that  warm  air  rises. 

3.  Radiation. — Two  bodies  not  in  contact  may  yet 
exchange  heat  across  the  intervening  space.  This 
transfer  is  effected  by  means  of  the  ether  of  space,  and 
is  called  radiation.  A  hot  stove  throws  the  ether  sur- 
rounding it  into  vibrations,  which  are  transmitted  ra- 
dially outward.  On  being  absorbed  (stopped)  by  bodies 
radiant  energy  is  again  converted  into  heat.*  The 

*The  Radiometer  is  an  instrument  that,  for  a  time,  was  supposed  to 
exhibit  the  actual  mechanical  force  of  the  sunbeam.  It  consists  of  a  tiny 


COMMUNICATION     OF     HEAT. 


259 


power  of  radiating  heat  and  of  absorbing  radiant  en- 
ergy differs  for  different  substances.  In  general,  good 
radiators  are  also  good  absorbers,  and  vice  versa.  The 
space  between  the  earth  and  the  sun  is  not  warmed  by 
the  sunbeam,  because  the  ether  of  space  is  incapable  of 
absorbing  radiant  energy.  Dry  air  is  so  poor  an  ab- 
sorber that  meat  can  be  cooked  by  radiation,  while  the 
surrounding  air  remains  at  the  freezing-point.  Aque- 
ous vapor  is  a  comparatively  good  absorber,  especially 
for  the  longer  waves.  For  this  reason  the  moisture  in 
the  atmosphere  stops  much  of  the  energy  of  the  sun- 
beam passing  through  it,  and  converts  it  into  heat. 
The  greater  part  of  the  solar  energy  absorbed  by  the 
earth  is  again  radiated  forth  into  space,  but  in  longer 
waves.  To  these  glass  is  especially  Fra* 185- 

opaque.*    Hence  its  use  in  the  con- 
struction of  green-houses.    At  lofty 


vane  delicately  pivoted  in  a  glass  globe  from 
which,  the  air  is  exhausted  as  fully  as  possible, 
when  the  globe  is  hermetically  sealed.  The  four 
arms  of  the  vane  carry  each  a  thin  light  disk  of 
mica  or  aluminium,  covered  with  lamp-black  on 
one  side  and  uncovered  on  the  other.  When  day- 
light falls  upon  it  the  little  vane  revolves  rapidly. 
The  motion  ceases  as  soon  as  the  light  is  cut  off. 
When  different  gases  are  admitted  into  the  globe, 
the  rate  of  rotation  varies.  It  is  now  believed 
that  the  unequal  heating  of  the  black  and  white 
surfaces  of  the  disks  causes  unequal  reaction 
of  the  molecules  of  air  left  in  the  vacuum. 
Lamp-black  being  the  best  absorber  and  radiator, 
receives  the  more  forcible  bombardment  of  flying 
air  molecules. 

*  In  the  course  of  Prof.  Langley's  experiments 
upon  Mount  Whitney,  water  was  boiled  by  expos- 
ing it  in  a  copper  vessel  covered  by  a  pane  of  win- 
dow-glass, to  the  direct  rays  of  the  sun.  This 


The  liiidiometer. 


260  HEAT. 

elevations  the  dry  air  allows  the  heat  received  by 
the  soil  during  the  day  to  escape  so  rapidly  that  a 
frost  occurs  before  morning,  though  the  heat  of  the 
afternoon  may  be  torrid. 

4.  Absorption  and  Reflection. — A  good  reflector 
is  a  poor  absorber  and  also  a  poor  radiator.  Snow 
is  a  good  reflector,  but  a  poor  absorber  or  radiator. 
Light  colors  often  absorb  solar  heat  less  and  re- 
flect more  than  dark  colors.*  White  is  generally 
considered  the  best  reflector,  and  black  the  best 
absorber  and  radiator.  But  the  nature  of  the  ma- 
terial is  of  more  importance  than  its  tint.  If  on  a 
bright  summer  day  three  thermometers  are  exposed 
to  the  sun,  one  held  up  in  mid-air,  another  resting 
on  a  bed  of  black  silk,  and  the  third  on  a  bed  of 
white  sand,  it  will  be  found  in  a  short  time  that  the 
temperatures  indicated  will  be  very  different.  The 
thermometer  on  the  sand  will  have  its  bulb  more 
warmed  than  that  on  the  bed  of  black  silk  ;  and  both 
of  these  will  be  warmer  than  the  one  in  mid-air. 

shows  that  many  of  the  heat-rays  of  the  sunbeam  are  stricken  down  by  the 
air  before  reaching  low  levels,  but  may  be  utilized  at  high  elevations.  So, 
Were  the  atmosphere  removed  the  earth  would  receive  far  more  heat  and  yet 
be  much  colder  than  now,  because  there  would  be  no  beds  of  water  vapor  to 
check  the  radiation  back  into  space.  See  "  American  Journal  of  Science," 
March,  1883. 

*  Experiments  show  that  with  artificial  heat  the  molecular  condition 
of  the  surface  varies  radiation  as  well  as  reflection.  In  fact,  white  lead  is 
as  good  a  radiator  as  lamp-black.— On  one  side  of  a  sheet  of  paper  paste  let- 
ters of  gold-leaf.  Spread  over  the  opposite  side  a  thin  coating  of  scarlet 
iodide  of  mercury— a  salt  which  turns  yellow  on  the  application  of  heat. 
Turn  the  scarlet  side  down.  Hold  over  the  paper  a  red-hot  iron.  The  gold- 
leaf  will  reflect  the  heat,  but  the  paper  spaces  between  the  letters  will 
absorb  it,  and  on  turning  the  paper  over,  the  gilt  letters  will  be  found 
traced  in  scarlet  on  a  yellow  background. 


THE     STEAM-ENGINE. 


261 


FIG.  186. 


IV.    THE   STEAM-ENGINE. 

WHEN  steam  rises  from  water  at  a  temperature 
of  212°,  it  has  an  elastic  force  of  nearly  15  Ibs.  per 
square  inch.  If  the  steam  be 
confined  and  the  temperature 
raised,  the  elastic  force  will  be 
rapidly  increased. 

1.  The  Steam-engine  is  a 
machine  for  using  the  elastic 
force  of  steam  as  a  motive 
power.  There  are  two  classes, 
high-^pressure  and  low-pressure. 
In  the  former,  the  steam,  after 
it  has  done  its  work,  is  forced 

OUt    into    the     air  ;     in    the    latter,       Steam-chest  and  Cylinder  of 

it    is    condensed    in    a    separate 

chamber  by  a  spray  of  cold  water.  As  the  steam 
is  condensed  in  the  low-pressure  engine,  a  vacuum 
is  formed  behind  the  piston  ;  while  the  piston  of  the 
high-pressure  engine  acts  against  the  pressure  of  the 
air.  The  elastic  force  of  the  steam  must  be  15  Ibs. 
per  square  inch  greater  in  the  latter  case.  The 
figure  represents  the  piston  and  connecting  pipes  of 
an  engine.  The  steam  from  the  boiler  passes 
through  the  pipe,  a,  into  the  steam-chest,  &,  as  indi- 
cated by  the  arrow.  The  sliding-valve  worked  by 
the  rod  h  lets  the  steam  into  the  cylinder,  alter- 
nately above  and  below  the  piston,  which  is  thus 
made  to  play  up  and  down  by  the  expansive  force. 
This  valve  is  so  arranged  that  at  the  moment  fresh 


262  HEAT. 

steam,  is  let  in  on  one  side  of  the  piston,  the  spent 
steam  on  the  other  side  is  released  into  the  outer 
air,  or  into  the  condensing  chamber. 

2.  The  Governor   is  an  apparatus  for  regulating 
the  supply  of  steam.      AB  is  the  axis  around  which 

iw  t^16  keavy  balls  E  and  D  revolve. 

They  are  so  connected  by  hinge- 
joints  that  the  ring  at  B  may  be 
pulled  down  or  lifted  by  them, 
while  that  at  C  is  fixed.  When 
trie  machine  is  going  too  fast,  the 
balls  fly  out  and  thus  pull  down 
the  rod,  K,  which  is  in  connection 
with  a  valve  that  controls  the  pipe 

The  Governor. 

supplying  steam.  A  portion  of  the 
steam  is  thus  obstructed,  and  the  revolution  of  the 
balls  becomes  slower.  This  in  turn  makes  them 
descend,  and  in  so  doing  they  lift  the  rod,  K.  The 
valve  is  thereby  opened  and  more  steam  supplied, 
whenever  the  speed  of  revolution  becomes  too  small. 

3.  A  High-pressure  Engine  is  shown  in  Fig.  188. 
A  represents  the  cylinder ;    B,  the  steam-chest  at  its 
side,  connected  with   it  on   the   interior  by  the  slid- 
ing-valve    already   shown ;    C,  the    throttle-valve    in 
the  pipe  through  which  steam  is  admitted  from  the 
boiler  ;    Z>,    the    governor ;     E,    the    band-wheel    by 
which  the  governor  is  driven  ;  F,  the  pump ;   G,  the 
crank ;    7,  the   conductor  attached  to  a,   the    cross- 
head ;  H,  the   eccentric  rod    (h  in   Fig.  186)    which 
works  the  sliding-valve  in  the  steam-chest ;  K,  the 


264  HEAT. 

governor-valve ;  S,  the  shaft  by  which  the  power  is 
conveyed  to  the  machinery.  The  cross-head,  a, 
slides  to  and  fro  in  a  groove,  and  is  fastened  to  the 
rod  which  works  the  piston  in  the  cylinder  A.  The 
expansive  force  of  the  steam  is  thus  communicated 
to  a,  thence  to  J,  by  which  the  crank  is  turned. 
The  heavy  fly-wheel  renders  the  motion  uniform 
(p.  23). 


V.    METEOROLOGY. 

1.  General  Principles. — (1.)  The  air  always  con- 
tains moisture.  The  amount  it  can  receive  depends 
upon  the  temperature ;  warm  air  absorbing  more, 
and  cold  air  less.  At  100°  F.,  a  cubic  foot  of  air 
can  hold  nearly  2  0  grains  of  invisible  water  vapor ; 
a  reduction  of  70°  will  cause  nine  tenths  of  that 
quantity  to  be  condensed  into  visible  droplets. 
When  the  air  at  any  temperature  contains  all  the 
vapor  it  can  hold  in  an  invisible  state,  it  is  said  to 
be  saturated ;  any  fall  of  temperature  will  then  con- 
dense a  part  of  the  vapor. 

(2.)  When  air  expands  against  pressure  (i.e.,  doing 
work  in  the  expansion),  its  energy,  being  thus  ex- 
pended, ceases  to  be  manifested  as  temperature. 
The  warm  air  from  the  earth  ascending  into  the 
upper  regions,  is  thus  rarefied  and  cooled.  Its  vapor 
is  then  condensed  into  clouds,  and  often  falls  as 
rain.  Owing  to  this  expansion  of  the  atmosphere 
and  the  greater  radiation  of  heat  in  the  dry  air  of 


METEOROLOGY.  265 

the  upper  regions,  there  is  a  gradual  diminution  of 
the  temperature  as  the  altitude  increases,  the  mean 
rate  in  the  north  temperate  zone  being  about  1°  for 
300  feet. 

2,  Dew.* — The  grass   at    night,  becoming  cooled 
by  radiation,  condenses  the  vapor  of  the  adjacent  air 
upon  its  surface.     Dew  will  gather  most  freely  upon 
the  best  radiators,  as  they  will  the  soonest  become 
cool.      Thus    grass,    leaves,    etc.,  receive    the   largest 
deposits.      It  will    not    form    on   windy  nights,    nor 
when  there  are  clouds  in  the  sky  to  reflect  the  heat 
radiated  from   the  ground.      In  tropical  regions  the 
nocturnal  radiation  on  clear  nights  is  often  so  great 
as  to  render  the  formation  of  ice  possible.    In  Bengal, 
water  is  exposed  for  this  purpose  in  shallow  earthen 
dishes  resting  on  rice  straw     In  parts  of  Chili,  Arabia, 
etc.,  by  its  abundance,  dew  feebly  supplies  the  place 
of  rain.    When  the  temperature  of  plants  falls  below 
32°,  the  vapor  is  frozen  upon  them  directly,  and  is 
called  white,  or  hoar-frost. 

3.  Fogs  are  formed  when  the  temperature  of  the 
air  falls  below  the  dew-point,  i.  e.,  the  temperature 
at  which  dew  is  deposited  for  a  given  degree  of  hu- 
midity.    They  are  characteristic  of  low  lands,  rivers, 
etc.,  where  the  air  is  saturated  with  moisture. 

*  Dew  was  anciently  thought  to  possess  wonderful  properties.  Baths 
in  this  precious  liquid  were  said  greatly  to  conduce  to  beauty.  It  was  col- 
lected for  this  purpose,  and  for  the  use  of  the  alchemists  in  their  weird 
experiments,  by  spreading  fleeces  of  wool  upon  the  ground.  Laurens,  a 
philosopher  of  the  middle  ages,  claimed  that  dew  is  ethereal,  so  that  if  we 
should  fill  a  lark's  egg  with  it  and  lay  it  out  in  the  sun,  immediately  on 
the  rising  of  that  luminary,  the  egg  would  fly  off  into  the  air  1 


266 


HEAT. 


4.  Clouds  differ  from  fogs  only  in  their  elevation 
in  the  atmosphere.  They  are  produced  chiefly  by 
the  cooling  due  to  expansion  as  currents  of  warm 
moist  air  rise  high  above  the  surface  of  the  ground. 
In  tropical  regions  they  float  only  at  great  heights ; 
in  arctic  regions,  near  the  groimd. 

FIG.  189. 


Different  kinds  of  Clouds— one  bird  indicates  the  Nimbus,  two  birds  the  Stratus, 
three  birds  the  Cumulus,  and  four  birds  the  Cirrus  cloud. 

The  stratus  cloud  is  composed  of  broad,  widely- 
extended  cloud-belts,  sometimes  spread  over  the 
whole  sky.  It  is  the  lowest  cloud,  and  often  rests 
on  the  earth,  where  it  forms  a  fog.  It  is  the  night- 
cloud. 

The  cumulus  cloud  is  made  up  of  large  cloud- 
masses  looking  like  snow-capped  mountains  piled  up 
along  the  horizon.  It  forms  the  summits  of  pillars 


METEOROLOGY.  267 

of  vapor,  which,  streaming  up  from  the  earth,  are 
condensed  in  the  upper  air.  It  is  the  day-cloud. 
When  of  small  size  and  seen  near  midday,  it  is  a 
sign  of  fair  weather. 

The  cirrus  (curl)  cloud  consists  of  light,  fleecy 
clouds  floating  high  in  air.  It  is  composed  of 
little  needles  of  ice  or  flakes  of  snow. 

The  cirro-cumulus  is  formed  by  small  rounded 
portions  of  cirrus  cloud,  having  a  clear  "sky  between. 
Sailors  call  this  a  "  mackerel  sky."  It  accompanies 
warm,  dry  weather. 

The  cirro-stratus  is  produced  when  the  cirrus 
cloud  spreads  into  long,  slender  strata.  It  forebodes 
rain  or  snow. 

The  cumulo-stratus  is  due  to  increase  in  thick- 
ness of  the  cumulus  clouds,  becoming  denser  and 
darker  below,  while  the  upper  parts  flatten  out  and 
thus  appear  like  the  stratus  clouds.  They  often  pre- 
cede thunder-storms. 

The  nimbus  cloud  is  that  from  which  rain  falls. 
It  may  be  produced  by  the  thickening  of  any  of  the 
forms  just  described. 

5.  Rain  is  the  product  of  rapid  condensation  of 
vapor  in  the  upper  regions.  At  a  low  temperature 
the  vapor  is  frozen  directly  into  snow.  This  may 
melt  before  it  reaches  the  earth,  and  fall  as  rain  or 
sleet.  A  sudden  draught  of  cold  air  into  a  heated 
ball-room  has  produced  a  miniature  snow-storm.  The 
wonderful  variety  and  beauty  of  snow-crystals  are 
illustrated  in  the  figure. 

Rain  always  warms  the  air.      Vapor  can  not  con- 


268  HEAT. 

dense  without  giving  out  as  heat  to  the  surrounding 
medium  the  energy  it  absorbed  in  assuming  the  va- 
porous state.*  It  has  been  estimated  that  the  heat  given 
to  the  west  coast  of  Ireland  by  rain-fall  is  equivalent 

FIG.  190. 


Snow-crystals. 

to  half  of  that  derived  from  the  sun.  At  Cherra- 
poonjee,  in  India,  the  annual  rain-fall  is  four  times 
as  great  as  on  the  coast  of  Ireland. 

6.  Winds  are  produced  by  variations  in  the  tem- 

*  "A  gallon  of  water  weighs  ten  pounds,  and  if  spread  out  so  as  to 
form  a  layer  an  inch  thick,  it  would  cover  about  two  square  feet  of  space. 
To  cover  a  square  mile  an  inch  in  depth,  60,000  tons  of  rain  are  required, 
or  12,000,000  gallons.  In  the  condensation  of  the  vapor  needed  to  produce 
a  single  gallon,  heat  enough  is  given  out  to  melt  75  pounds  of  ice,  or  to 
make  45  pounds  of  cast-iron  white-hot.  An  inch  of  rain-fall  on  each  square 
mile  hence  implies  an  evolution  of  heat  sufficient  to  melt  a  layer  of  ice 
spread  over  the  ground  8  inches  thick,  or  to  liquefy  a  globe  of  iron  130 
feet  in  diameter,  or  a  rod  of  it  a  foot  in  thickness  and  260  miles  in 
length." 


METEOROLOGY.  269 

perature  of  the  air.  The  atmosphere  at  some  point 
is  heated  and  expanded;  it  rises  and  colder  air  flows 
in  to  supply  its  place.  This  produces  currents.  The 
land  and  sea  breezes  of  tropical  islands  are  caused 
by  the  unequal  specific  heat  of  land  and  water.  Dur- 
ing the  day  the  land  becomes  more  highly  heated 
than  the  water,  and  hence  toward  noon  a  sea-breeze 
sets  in  from  the  ocean,  and  is  strongest  in  the  after- 
noon. At  night  the  land  cools  faster  than  the  wa- 
ter, and  so  a  land-breeze  sets  out  from  the  land,  and 
is  strongest  after  midnight. 

Trade-winds  are  so  named  because  by  their  regu- 
larity they  favor  commerce.  A  vessel  on  the  Atlantic 
Ocean  will  sometimes,  without  shifting  a  sail,  set 
steadily  before  this  wind  from  Cape  Verde  to  the 
American  coast.  The  air  about  the  equator  is  highly 
heated,  and,  rising  to  the  upper  regions,  flows  off 
north  and  south.  The  cold  air  near  the  poles  sets 
toward  the  equator  to  fill  its  place.  If  the  earth 
were  at  rest  this  would  make  an  upper  current 
flowing  from  the  equator,  and  a  lower  current  flow- 
ing toward  it.  As  the  earth  is  rotating  on  its  axis 
from  west  to  east,  the  under  current  starting  from 
the  poles  is  constantly  coming  to  a  part  moving 
faster  than  itself.  It  therefore  lags  behind.  When 
it  reaches  the  north  equatorial  regions,  it  lags  so 
much  that  it  becomes  a  current  from  the  north-east, 
and  in  the  south  equatorial  regions  a  current  from 
the  south-east. 

7.  Ocean  Currents  are  produced  in  a  similar 
manner.  The  water  heated  by  the  vertical  sun  of 


270  HEAT. 

the  tropics  rises  and  flows  toward  the  poles.  The 
Gulf  Stream  carries  the  heat  of  the  Caribbean  Sea 
across  the  Northern  Atlantic  to  the  shores  of  Scot- 
land and  Norway.  This  great  stream  of  warm  water, 
flowing  steadily  through  the  cold  water  of  the  ocean, 
rescues  England  from  the  snows  of  Labrador.  Were 
it  not  for  the  barrier  of  a  chain  of  mountains  con- 
necting North  and  South  America,  Great  Britain 
would  be  condemned  to  arctic  glaciers. 

8.  Adaptations  of  Water. — The  great  specific 
heat  of  water  exercises  a  marked  influence  on 
climate.  It  tends  to  prevent  sudden  changes  of 
weather.  In  the  summer  it  absorbs  vast  quantities 
of  heat,  which  it  gives  off  in  the  fall,  and  thus  mod- 
erates the  approach  of  winter.  In  the  spring  the 
melting  ice  and  snow  drink  in  the  warmth  of  the 
sunbeam.  Since  so  much  heat  is  required  to  melt 
the  ice  and  snow,  they  dissolve  very  slowly,  and  thus 
ward  off  the  disastrous  floods  which  would  follow, 
if  they  passed  quickly  into  the  liquid  state. 

Water  contains  air,  which  is  necessary  for  the 
support  of  animal  life.  This  air  not  only  makes  it 
available  as  a  home  for  fish  and  other  creatures  that 
inhabit  the  water,  but  also  makes  the  change  from 
water  to  steam  more  gradual.  Much  of  it  is  driven 
off  when  the  water  is  heated.  When  water  has  been 
carefully  deprived  of  the  air  usually  held  in  solution, 
it  is  liable  to  violent  commotion  at  any  moment 
when  it  is  heated  above  212°  F.  With  such  water 
every  stove-boiler  would  need  a  thermometer.  A 
tea-kettle  would  require  as  careful  watching  as  a 


METEOROLOGY.  271 

steam-engine,  and  our  kitchens  would  witness    fre- 
quent and  perhaps  disastrous  explosions. 

Water,  like  other  liquids,  expands  when  heated  and 
contracts  when  cooled  for  any  temperatures  above 
39.2°  F.  The  density  of  water  is  greatest  at  this  tem- 
perature. Cooled  below  39.2°  F.  water  expands  till 
it  becomes  solid  at  32°  F.  One  volume  at  39.2°  F. 
becomes  1.00013  volumes  at  32°  F.*  This  expan- 
sion in  cooling  is  probably  due  to  the  regrouping  of  the 
molecules  of  the  water  preparatory  to  crystallization, 
the  crystalline  structure  requiring  more  space  than 
the  liquid  form.  As  soon  as  a  few  crystals  are 
definitely  formed,  each  serves  as  a  nucleus  around 
which  others  gather,  and  the  process  becomes  then 
far  more  rapid.  Certain  metals,  such  as  bismuth 
and  iron,  act  like  water  in  this  respect,  and  are 
hence  well  fitted  for  making  sharp  castings,  filling 
every  crevice  of  the  mold  as  they  expand  in  crys- 
tallizing, f  This  crystallization  of  water  is  of  great 
importance  in  connection  with  the  freezing  of  our 
lakes  and  rivers.  Were  it  not  crystalline  when 

*  Since  ice  when  it  melts  contracts,  pressure  aids  in  liquefaction  and 
so  lowers  the  melting-point.  In  descending  over  the  rough  surface  of  a 
mountain  slope,  glacier  ice  is  subjected  to  alternate  pressure  and  extension. 
The  pressure  melts  it  and  makes  the  mass  slide  farther  down.  Passing  over 
some  ledge  it  snaps,  producing  great  fissures.  When  the  walls  of  these 
come  into  contact  they  freeze  together  again,  but  only  to  be  re-melted. 

t  Fit  a  small  flask  with  a  cork,  through  which  passes  an  upright  glass 
tube.  Fill  with  colored  water.  Apply  heat  to  the  flask  until  the  liquid 
runs  over  the  top  of  the  tube.  This  shows  the  expansion  by  heat.  Now 
apply  a  freezing  mixture  to  the  flask,  and  at  first  the  liquid  in  the  tube 
falls,  but  soon  begins  to  rise.  When  it  runs  over  as  before,  apply  heat 
and  it  shrinks  back  again.  Thus  cold  will  expand  and  heat  contract  it. 
When  water  is  at  its  maximum  density  (about  39°)  expansion  sets  in 
alike,  whether  you  heat  or  cool  it. 


272  HEAT. 

frozen,  the  water  at  the  surface  during  severe 
weather,  radiating  its  heat  and  becoming  chilled, 
would  contract  and  fall  to  the  bottom,  while  the 
warm  water  below  would  rise  to  the  top.  This 
process  would  continue  until  the  freezing-point  was 
reached,  when  the  whole  mass  would  solidify  into 
ice.  Our  lakes  and  rivers  would  freeze  solid  every 
winter.  This  would  be  fatal  to  all  animal  life  in  it, 
at  least  of  the  higher  orders,  such  as  fish.  In  the 
spring  the  ice  would  not,  as  now,  buoyant  and  light, 
float  and  melt  in  the  direct  sunbeam,  but,  lying  at 
the  bottom,  would  be  protected  by  the  non-conduct- 
ing water  above.  The  longest  summer  would  not  be 
sufficient  to  thaw  the  deeper  bodies  of  water.  As  it 
is,  the  ice  is  formed  at  the  surface,  and  there  it  floats, 
protecting  the  water  beneath  from  further  reduction 
of  temperature.* 

Water,  in  freezing,  has  a  tendency  to  free  itself 
from  salts  and  other  substances  dissolved  in  it. 
Thus,  melting  ice  furnishes  a  means  of  obtaining 
fresh  water  in  Arctic  regions.  If  a  barrel  of  vinegar 
freeze,  we  shall  find  much  of  the  acid  collected  in  a 
mass  about  the  center  of  the  ice. 

*  Water  distills  from  the  ocean  and  land  as  vapor,  at  one  time  cooling 
and  refreshing  the  air,  at  another  moderating  its  wintry  rigor.  It  con- 
denses into  clouds,  which  shield  the  earth  from  the  direct  rays  of  the  sun, 
and  protect  against  excessive  radiation.  It  falls  as  rain,  cleansing  the  air 
and  quickening  vegetation  with  renewed  life.  It  descends  as  snow,  and, 
like  a  coverlet,  wraps  the  grass  and  tender  buds  in  its  protecting  em- 
brace. It  bubbles  up  in  springs,  invigorating  us  with  cooling,  healing 
draughts  in  the  sickly  heat  of  summer.  It  purifies  our  system,  dissolves 
our  food,  and  keeps  our  joints  supple.  It  flows  to  the  ocean,  fertilizing 
the  soil,  and  floating  the  products  of  industry  and  toil  to  the  markets  of 
the  world.  (See  "  Chemistry,"  p.  56-63.) 


PRACTICAL     QUESTIONS.  273 


PRACTICAL     QUESTIONS. 

1.  "Why  will  one's  hand,   on    a   frosty  morning,  freeze  to  a  metallic 
door-knob  sooner  than  to  one  of  porcelain  ? 

2.  Why  does  a  piece  of  bread  toasting  curl  up  on  the  side  exposed  to 
the  fire  ? 

3.  Why  do  double  windows  protect  better  than  single  ones  from  the 
cold? 

4.  "Why  do  furnace-men  wear  flannel  shirts  in  summer  to  keep  cool, 
and  in  winter  to  keep  warm? 

5.  Why  do  we  blow  our  hands  to  make  them  warm,  and  our  soup  to 
make  it  cool? 

6.  Why  does  snow  protect  the  grass  in  winter? 

7.  Why  does  water  "boil  away"  more  rapidly  on  some  days  than  on 
others? 

8.  What  causes  the  crackling  sound  of  a  stove  when  a  fire  is  lighted? 

9.  Why  is  the  tone  of  a  piano  higher  in  a  cold  room  than  in  a  warm 
one? 

10.  Ought  an  inkstand  to  have  a  large  or  a  small  mouth? 

11.  Why  is  there  a  space  left  between  the  ends  of  the  rails  on  a  rail- 
road track? 

12.  Why  is  a  person  liable  to. take  cold  when  his  clothes  are  damp? 

13.  What  is  the  theory  of  corn-popping? 

14.  Could  vacuum-pans  be  employed  in  cooking? 

15.  Why  does  the  air  feel  so  chilly  in  the  spring,  when  snow  and  ice 
are  melting? 

16.  Why,  in  freezing  ice-cream,  do  we  put  the  ice  in  a  wooden  vessel 
and  the  cream  in  a  tin  one? 

17.  Why  does  the  temperature  generally  moderate  when  snow  falls? 

18.  What  causes  the  singing  of  a  tea-kettle?     Aw.  The  escaping  steam 
is  thrown  into  vibration  by  friction  against  the  spout. 

19.  Why  does  sprinkling  a  floor  with  water  cool  the  air? 

20.  How  low  a  degree  of  temperature  can  be  marked  by  a  mercurial 
thermom  eter  ? 

21.  If  the  temperature  is  70°  R,  what  is  it  C.? 

22.  Will  dew  form  on  an  iron  bridge?    On  a  plank  walk? 

23.  Why  will  not  corn  pop  when  very  dry? 

24.  When  the  interior  of  the  earth  is  so  hot,  why  do  we  get  the  coldest 
water  from  a  deep  well? 

25.  Ought  the  bottom  of  a  tea-kettle  to  be  polished? 

26.  Which  boils  the  sooner,  milk  or  water? 

27.  Is  it  economy  to  keep  our  stoves  highly  polished? 

28.  If  a  thermometer  be  held  in  a  running  stream,  will  it  indicate  the 
same  temperature  that  it  would  in  a  pailful  of  the  same  water? 

29.  Which  makes  the  better  holder  when  one  wishes  to  protect  his 
hands  from  a  hot  dish,  woolen  or  cotton? 


274  HEAT. 

30.  Which  will  give  out  the  more  heat,  a  plain  stove  or  one  with  orna- 
mental designs? 

31.  Does  dew  fall? 

32.  What  causes  the  "sweating"  of  a  pitcher? 

33.  Why  is  evaporation  hastened  in  a  vacuum? 

34.  Does  stirring  the  ground  around  plants  aid  in  the  deposition  of 
dew? 

•  35.  Why  does  the  snow  at  the  foot  of  a  tree  melt  sooner  than  that  in 
tho  open  field? 

36.  Why  is  the  opening  in  a  chimney  made  to  decrease  in  size  from 
bottom  to  top? 

37.  Will  tea  keep  hot  longer  in  a  bright  or  a  dull  tea-pot? 

38.  What  causes  the  snapping  of  wood  when  laid  on  the  fire  ?    Am.  The 
expansion  of  the  air  in  the  cells  of  the  wood. 

39.  Why  is  one's  breath  visible  on  a  cold  day? 

40.  What  gives  the  blue  color  to  air?     Am.    The  particles  floating  in  it 
reflect  the  blue  light  of  the  sunbeam. 

41.  How  does  the  heat  at  two  feet  from  the  fire  compare  with  that  at 
four  feet? 

42.  Why  does  the  frost  remain  later  in  the  morning  upon  some  objects 
than  upon  others? 

43.  Is  it  economy  to  use  green  wood? 

44.  Why  does  not  green  wood  snap? 

45.  Why  will  a  piece  of  metal  dropped  into  a  glass  or  porcelain  dish  of 
boiling  water  increase  the  ebullition? 

46.  Which  can  be  ignited  the  more  quickly  with  a  burning-glass,  lamp- 
black or  white  paper? 

47.  Why  does  the  air  feel  colder  on  a  windy  day? 

48.  Could  a  burning-lens  be  made  of  ice? 

49.  Why  is  an  iceberg  frequently  enveloped  by  a  fog? 

50.  Would  dew  gather  more  freely  on  a  rusty  stove  than  on  a  bright 
kettle  ? 

51.  Why  is  a  clear  night  colder  than  a  cloudy  one  during  the  same 
season  ? 

52.  Why  is  no  dew  formed  on  cloudy  nights  ? 

53.  Why  will  "fanning"  cool  the  face? 

54.  How  are  safes  made  fire-proof? 

55.  Why  can  you  heat  water  more  quickly  in  a  tin  than  a  china  cup? 

56.  Why  will  a  woolen  blanket  keep  ice  from  melting? 

57.  Does  dew  form  under  trees? 

58.  What  is  the  principle  of  heating  by  steam? 

59.  What  is  the  cause  of  "cloud-capped"  mountains? 

60.  Show  how  the  glass  in  a  hot-house  acts  as  a  trap  to  catch  the  sun- 
beam. 

61.  Does  the  heat  of  the  sun  come  in  through  our  windows? 

62.  Does  the  heat  of  our  stoves  pass  out  in  the  same  way? 

03.  The  top  of  a  mountain  is  nearer  the  sun,  why  is  it  not  warmer? 


SUMMARY.  275 

64.  What  is  hoar-frost?    Ans.    Frozen  dew. 

65.  "Why  will  a  slight  covering  protect  plants  from  frost?  Ans.  Because 
it  prevents  radiation. 

66.  Why  is  there  no  frost  on  cloudy  nights?    Ans.    The  clouds  act  like 
a  blanket,  to  prevent  radiation  and  keep  the  earth  warm. 

67.  Can  we  find  frost  on  the  windows  and  on  the  stone  flagging  the 
same  morning? 

68.  Why  will  not  snow  "pack  "  into  halls  except  in  mild  weather? 

69.  Why  is  the  sheet  of  zinc  under  a  stove  so  apt  to  become  puckered? 

70.  Why  does  a  mist  gather  in  the  receiver  of  the  .^-pump  as  the  air 
becomes  rarefied? 

71.  Why  are  the  tops  of  high  mountains  in  the  tropics  covered  with 
perpetual  snow? 


SUMMARY. 

Heat  is  produced,  by  longer  and  less  refrangible  waves  and 
slower  vibrations  of  ether  than  those  which  cause  light.  Solar 
energy  may  be  radiated,  reflected,  refracted,  absorbed,  and  po- 
larized, whether  manifested  as  light  or  heat.  If  we  elevate 
the  temperature  of  a  body  sufficiently,  such  as  a  piece  of  plati- 
num, we  can  cause  it  to  emit  rays  of  both  heat  and  light.  A 
body  which  allows  the  radiant  heat  to  pass  through  it  easily 
is  styled  diathermanous ;  rock-salt  is  such  a  body,  being  to 
heat-rays  what  glass  is  to  light-rays.  The  sun  is  the  principal 
source  of  heat.  But  heat  can  be  obtained  by  chemical  and 
mechanical  means.  In  burning  coal  we  secure  it  by  the  former 
method.  Mechanical  energy  may  be  changed  directly  into  heat, 
as  in  striking  fire  with  flint  and  steel,  and  in  hammering  a 
bullet  on  an  anvil  until  it  is  hot.  According  to  Joule's  law, 
772  feet  fall  of  a  given  weight  corresponds  to  1°  of  rise  of  tem- 
perature in  the  same  weight  of  water. 

Among  the  physical  effects  of  heat  are  a  change  of  temper- 
ature, expansion,  liquefaction,  vaporization,  and  evaporation. 
The  heat-force  increases  the  kinetic  energy  of  the  molecules, 
thus  elevating  the  temperature ;  and  the  increased  vibration  of 
the  molecules  causes  an  expansion  of  the  body.  The  latter  is 
so  uniform  in  certain  substances,  such  as  mercury,  that  it  is 
used  to  indicate  changes  of  temperature,  as  in  the  thermometer. 
The  expansion  of  the  metals  by  heat  is  turned  to  account  in 


276  HEAT, 

many  art  processes.  The  walls  of  a  gallery  in  the  Conservatoire 
des  Arts  et  Metiers  in  Paris,  had  begun  to  bulge.  To  remedy 
this,  iron  rods  were  passed  across  the  building  and  screwed  into 
plates  on  the  outside  of  the  walls.  By  heating  the  bars,  they 
were  expanded,  when  they  were  screwed  up  tightly.  Being 
then  allowed  to  cool,  they  contracted,  thus  drawing  the  walls 
back  toward  a  perpendicular.  The  same  has  been  done  for 
weakened  walls  in  many  other'  places. 

Heat  is  the  great  antagonist  of  cohesion.  The  liquid  and  gas- 
eous states  of  bodies  depend  on  its  relative  presence  or  absence 
(absolute  cold  is  as  yet  only  a  theoretical  condition,  all  bodies 
with  which  we  are  familiar  being  relatively  warm).  When  the 
heat-force  nearly  balances  that  of  cohesion,  the  body  breaks 
down  into  a  liquid,  and  when  the  repellent  fairly  triumphs,  the 
particles  fly  off  as  a  gas.  Immediately  before  and  after  each 
of  these  marked  changes,  viz.,  of  a  solid  to  a  liquid  and  of  a 
liquid  to  a  gas,  the  thermometer  indicates  a  constant  temper- 
ature. Thus  water  from  melting  ice  affects  the  thermometer 
just  as  the  ice  does,  and  steam  is  no  hotter  than  the  boiling 
water.  The  heat  which,  in  these  processes,  becomes  hidden 
from  the  thermometer  is  called  latent,  though  we  now  know 
that,  having  been  occupied  in  doing  internal  work,  it  has  merely 
become  potential,  and  can  be  readily  turned  again  into  kinetic 
energy.  The  so-called  latent  heat  of  water  is  only  the  potential 
heat-energy  of  the  separated  molecules,  which  will  reappear  the 
instant  the  molecules  collapse  and  come  once  more  within  the 
grasp  of  cohesion.  On  this  principle  is  based  the  method  of 
heating  by  steam.  Evaporation  is  a  slow  change  to  vapor  that 
takes  place  at  all  temperatures,  but  may  be  greatly  increased 
by  a  diminution  of  pressure,  as  in  a  vacuum.  It  is  a  cooling 
process,  and  is  practically  applied  to  the  manufacture  of  ice. 

By  the  subtraction  of  heat,  i.  e.,  by  cold,  and  by  the  addi- 
tion of  pressure,  which  antagonizes  the  repellent  heat-force, 
gases  may  be  liquefied  and  even  congealed,  the  transparent  car- 
bonic-acid gas  thus  becoming  a  snowy  solid.  What  were  for- 
merly called  the  "permanent  gases"  (oxygen,  hydrogen,  etc.), 
have  been  liquefied  by  means  of  the  cold  produced  by  their 
rarefaction  when  they  were  suddenly  released  from  a  pressure 
of  two  or  three  hundred  atmospheres. 


HISTORICAL     SKETCH.  277 

Heat  is  conducted  from  molecule  to  molecule  of  a  body, 
radiated  in  straight  lines  through  air  (or  space),  and  circulated 
by  the  transference  of  heated  masses  through  a  change  of  spe- 
cific gravity  due  to  expansion.  The  first  method  is  character- 
istic of  solids,  and  the  third  of  liquids  and  gases.  The  elastic 
force  of  steam  increases  when  it  is  confined  and  a  higher  tem- 
perature is  reached.  The  steam-engine  utilizes  this  principle. 
There  are  two  forms  of  this  machine,  the  high-pressure  and  the 
low-pressure,  according  as  the  waste  steam  is  ejected  into  the 
air  or  condensed  in  a  separate  chamber.  The  phenomena  of 
dew,  rain,  etc.,  depend  upon  the  fact  that  a  change  from  a 
higher  to  a  lower  temperature  causes  the  air  to  deposit  its 
moisture. 


HISTORICAL      SKETCH. 

DEMOCRITUS,  the  originator  of  the  Atomic  Theory,  held  that 
heat  consists  of  minute  spherical  particles  radiated  rapidly 
enough  to  penetrate  every  substance.  Until  very  recently,  heat 
and  light  were  thus  reckoned  among  the  Imponderables,  i.  e., 
matter  which  has  no  weight.  Aristotle  considered  heat  more  a 
condition  than  a  substance.  Bacon,  in  his  "Novum  Organum," 
wrote :  "Heat  is  a  motion  of  expansion."  Locke,  half  a  century 
later,  said:  "Heat  is  a  very  brisk  agitation  of  the  insensible 
parts  of  an  object,  which  produces  in  us  the  sensation  from 
whence  we  denominate  the  object  hot,  so  that  what  in  our 
sensation  is  heat,  in  the  object  is  motion." 

The  material  view,  however,  held  its  ground.  At  the  begin- 
ning of  the  18th  century,  Stahl  elaborated  a  theory  that  a 
buoyant  substance  called  phlogiston  is  the  principle  of  heat,  and 
that  when  a  body  burns,  its  phlogiston  escapes  as  fire.  In 
1760,  Dr.  Black  investigated  and  made  known  the  principles  of 
what  he  termed  latent  heat,  i.  e.,  heat  which  becomes  hidden 
when  ice  is  turned  into  water  or  water  into  steam.  Priestley 
discovered,  in  1774,  and  Lavoisier  afterward  developed,  the 
modern  view  of  combustion.  But  the  latter  philosopher  then 
advanced  the  theory  that  heat  (caloric)  is  an  actual  substance, 
which  passes  freely  from  one  body  to  another  and  combines  at 


278  HEAT. 

pleasure.  Toward  the  close  of  the  18th  century,  Benjamin 
Thompson,  better  known  as  Count  Rumford,  a  native  of  Wo- 
burn,  Mass.,  but  in  the  employ  of  the  Elector  of  Bavaria, 
proved  the  convertibility  of  force.  "He  first  took  the  subject," 
as  Professor  Youmans  well  remarks,  "out  of  the  domain  of 
metaphysics,  where  it  had  been  speculated  upon  since  the  time 
of  Aristotle,  and  placed  it  on  the  true  basis  of  physical  experi- 
ment." 

Soon  the  scientific  world  seemed  to  be  ripe  for  this  dis- 
covery, and  it  appears  to  have  sprung  up  spontaneously  in 
men's  thoughts  every-where.  Mayer,  a  physician  of  Germany, 
and  Grove,  of  England,  proved  the  mutual  relation  of  the 
forces,  the  latter  first  using  the  term  "  Correlation  of  Forces," 
since  changed  to  Conservation  of  Energy.  Joule  discovered  the 
law  of  the  "Mechanical  Equivalent  of  Heat,"  about  1843.  In 
his  famous  experiments,  he  used  pound-weights  made  to  fall 
through  a  measured  distance.  Cords  were  attached  to  them,  so 
that,  as  they  fell,  they  turned  a  paddle-wheel  placed  in  a  box  of 
water.  Other  liquids  were  used  instead  of  the  water.  The  rise 
of  temperature  in  the  liquids  was  carefully  marked.  The  loss 
by  friction  in  the  apparatus  was  estimated,  and  so,  at  last,  the 
dynamical  theory  of  heat  was  fully  demonstrated.  Names  of 
philosophers  well  known  to  us,  such  as  Henry,  Helmholtz, 
Faraday,  Thomson,  Maxwell,  Le  Conte,  Youmans,  Stewart,  and 
Tyndall,  are  associated  with  the  final  establishment  of  this 
theory. 

Consult,  on  this  interesting  subject,  Tait's  "  Recent  Advances 
in  Physical  Science";  Stewart's  "Treatise  on  Heat";  Tyndall's 
"Heat  a  Mode  of  Motion";  MaxweU's  "Theory  of  Heat"; 
Thurston's  "  History  of  the  Growth  of  the  Steam-engine " ; 
Buckley's  "Short  History  of  Natural  Science'';  Smiles'  "Lives 
of  Boulton  and  Watt"  ;  Youmans'  "Correlation  of  the  Physical 
Forces";  "Read  and  the  Steam-engine";  "American  Cyclope- 
dia," Art.  "Steam-engine";  "Popular  Science  Monthly,"  Vol. 
XII.,  p.  616,  Art.  "Liquefaction  of  Gases";  Scott's  "Meteor- 
ology," and  Thomson's  "Cruise  of  the  Challenger." 


IX. 

MAGNETISM. 


'  THAT  power  which,  like  a  potent  spirit,  guides 
The  sea-wide  wanderers  over  distant  tides, 
Inspiring  confidence  where'er  they  roam, 
By  indicating  still  the  pathway  home;  — 
Through  Nature,  quickened  by  the  solar  beam, 
Invests  each  atom  with  a  force  supreme, 
Directs  the  cavern'd  crystal  in  its  birth, 
And  frames  the  mightiest  mountains  of  the  earth 
Each  leaf  and  flower  by  its  strong  law  restrains 
And  binds  the  monarch  Man  within  its  mystic  chains." 

HUNT. 


ANALYSIS  OF  MAGNETISM. 


MAGNETISM.  . 


1.  MAGNETS. 

2.  THE  MAGNETIC  MERIDIAN. 

3.  LAWS  OF  MAGNETISM. 

4.  INDUCTION. 

5.  How  TO  MAKE  A  MAGNET. 

6.  THE  COMPASS. 

7.  LINES  OF  FORCE. 

8.  POLARITY  OF  THE  NEEDLE. 

9.  TERRESTRIAL  MAGNETISM. 


MAGNETISM. 

1.  Magnets.* — A  natural  magnet  is  an  ore  6f  iron 
(Fe304,  "Popular  Chemistry,"  p.  156),  generally  known 


FIG.  191. 


Magnet  Dipped  in  Iron  Pilings 


as  lodestone   (Saxon,  laedan,  to  lead),  which  has  the 
power  of  attracting  iron.f    The  artificial  magnet  is  a 

*  The  term  is  derived  from  the  fact  that  an  ore  of  iron  possessing  this 
property  was  first  found  at  Magnesia,  in  Asia  Minor. 

t  A  few  other  elements,   such  as   nickel  and  cobalt,   are    attracted 


282 


MAGNETISM. 


steel  bar  that  has  acquired  properties  like  those  of 
lodestone.  If  it  be  straight,  it  is  called  a  bar  mag- 
net; if  U-shaped,  a  horseshoe  magnet.  A  piece  of 
soft  iron  called  the  armature  is  placed  so  as  to  con- 
nect the  two  ends  of  the  horseshoe. 

If  we  insert  a  magnet  in  iron  filings,  they  will 
cling  chiefly  to  its  ends  termed  the  poles.  The 
magnetic  force  will  be  exerted  even  through  any  in- 
tervening body  that  is  not  itself  magnetic. 

FIG.  192. 


Influence  of  one  Magnet  on  another. 

2.  The  Magnetic  Meridian.— If  a  slender  bar 
magnet  be  suspended  so  as  to  swing  freely  in  a 
horizontal  plane,  it  will  come  to  rest  in  a  definite 
position  ;  one  pole  pointing  north,  the  other  south.  A 
vertical  plane  passing  through  the  two  poles  in  this 

slightly  by  the  magnet.    They  are  all  called  magnetic  bodies,  but  for  ordi- 
nary purposes  iron  may  be  regarded  as  the  only  one  of  importance. 


INDUCTION.  283 

position  is  called  the  magnetic  meridian.  The  pole 
pointing  north  is  called  the  north  or  (-h)  pole,  the 
other  the  south  or  (— )  pole. 

3.  Laws  of  Magnetism.— If  we  hold  a  magnet 
near  a  magnetic  needle,  we  shall  find  that  the  south 
pole  of  one  attracts  the  north  pole,  and  repels  the 
south  pole  of  the  other.*  This  proves  the  law— 

"  Like  poles  repel,  and 

FIG.  193. 

unlike  poles  attract. 

Two  opposite  poles 

Magnetic  induction.      ^^     placed    near   together 

attract      each      other 

strongly,  but  this  force,  like  gravitation,  dimin- 
ishes as  the  square  of  the  distance  increases. 

4.  Induction  is  the  process  of  developing 
magnetism  by  bringing  a  magnetic  body  and  a  mag- 
net near  together.  If  a  piece  of  soft  iron  be  brought 
near  a  magnet,  it  immediately  assumes  the  magnetic 
state,  but  loses  it  on  being  removed.  In  steel  the 

*  Experiments.—  1.  Rub  the  point  of  a  sewing-needle  across  the  north 
pole  of  a  magnet.  Bring  the  point  near  the  south  pole  of  the  magnetic 
needle.  The  needle  will  be  repelled,  showing  that  the  point  of  the  sewing- 
needle  has  become  a  south  pole.  2.  Suspend  a  key  from  the  north  pole  of 
a  magnet.  Bring  the  south  pole  of  an  equal  magnet  close  to  the  upper  end 
of  the  key.  The  key  will  instantly  fall.  3.  Suspend  a  long  iron  wire  from 
the  north  pole  of  a  magnet.  Bring  the  north  pole  of  the  second  magnet 
near  the  lower  end  of  the  wire.  The  wire  is  repelled,  because  its  lower 
extremity  possesses  north  polarity.  4.  Immerse  the  unlike  poles  of  two 
magnets  in  iron  filings.  Bring  the  two  poles  near  each  other.  The  filings 
will  move  toward  one  another.  But  if  the  poles  of  the  magnets  are  like, 
the  filings  will  fall  off  the  magnets.  5.  To  ascertain  whether  a  metallic 
substance  contains  iron :  Bring  the  substance  near  one  of  the  extremities 
of  a  magnetic  needle.  If  the  position  of  the  needle  be  affected,  then  the 
substance  almost  certainly  contains  Iron.  A  piece  of  copper  will  not  affect 
the  magnetic  needle. 


284  MAGNETISM. 

change  is  induced  and  lost  much  more  slowly.  The 
end  of  the  bar  next  to  the  south  pole  of  the  magnet 
becomes  the  north  pole  of  the  new  magnet,  and  vice 
versa.  When  opposite  states  are  thus  developed  in 
the  opposite  ends  of  a  body,  it  is  said  to  be  polarized. 
Whenever  an  object  is  attracted  by  a  magnet,  it  is 
supposed  first  to  be  made  a  magnet  (polarized)  by 
induction,  and  then  the  attraction  consists  in  that  of 
unlike  poles  for  each  other.  Thus  we  may  suspend 
from  a  magnet  a  chain  of  rings  held  together  by 
magnetic  attraction.*  Each  link  is  a  magnet  with 
its  north  and  south  poles.  Each  particle  of  the  tuft 
of  filings  in  Fig.  191  is  a  distinct  magnet.  By  in- 
ducing magnetism,  a  magnet  does  not  lose  force.  It 
rather  gains  by  the  reciprocal  influence  of  the  new 
magnet.  An  armature  acts  in  this  manner  to 

strengthen   a   magnet.      If 

we  break  a  ma£net'  the 
smallest    fragment 


9  IP  i«  *  i« i«i<  n  mi  B  B  u  x«^g  i«la»i«  I* 


Polarization  of  an  Iron  Bar. 

have  a  north  and  a  south 

pole.  This  is  explained  by  supposing  that  every 
molecule  contains  two  opposite  kinds  of  energy  which 
neutralize  each  other.  When  the  bar  is  magnetized 
these  are  separated,  but  do  not  leave  the  molecule. 
This  is  hence  polarized,  the  halves  assuming  opposite 
magnetic  states,  as  shown  in  Fig.  194.  The  light  half 
of  each  little  circle  represents  the  positive,  and  the 
dark  the  negative  side.  All  the  molecules  exert  their 
negative  force  in  one  direction,  and  their  positive  in 

*  Repeat  this  experiment  with  keys,  or  nails  of  different  sizes,  or  bits 
of  wire  of  varying  length. 


LINES     OF     FORCE.  285 

the  other.  The  forces  thus  neutralize  each  other  at 
the  center,  but  manifest  themselves  at  the  ends  of 
the  magnet.  Hence  it  is  impossible  PIG  195 

to  produce  a  magnet  with  only  one 
pole.  Each  pole  necessitates  the  pres- 
ence of  the  other. 

5.  How   to    Make    a    Magnet. — 
Place  the  inducing  magnet,  as  shown       ^"H 

in  Fig.  195,  on  the  unmagiietized  bar     Makiug  a  Magnet 

(which  any  blacksmith  can  make  from 

a  bar  of  steel),  and  draw  it  from  right  to  left  several 

times,  always  carrying  it  back  through  the  air  to  the 

starting-point.* 

6.  The  Compass  is  a  magnetic  needle  used  by 
mariners,  surveyors,  etc.     It  is  delicately  poised  over 
a  card  on  which  the  "points  of  the  compass"  are 
marked.      At  most  places  the  needle  does  not  point 
directly  N.  and  S.    The  "  line  of  no  variation  "  in  the 
United  States  passes  near  Wilmington,  N.  C.,  Char- 
lottesville,  Va.,  and  Pittsburg,  Pa,      East  of  this  the 
declination  of  the  needle  from  true  north  is  toward 
the  west,   and  west  of  it  the  declination  is  toward 
the  east. 

7.  Lines  of  Force. — The  region  in  the  neighbor- 
hood of  a  magnet   pole  within  which  it  can  have  a 
perceptible  effect  upon  magnetic  bodies  is  called  its 

*  A  needle  may  be  magnetized  by  laying  it  across  the  poles  of  a  horse- 
shoe magnet.  After  remaining  a  few  moments,  the  end  in  contact  with  the 
north  pole  of  the  magnet  will  become  a  south  pole  and  the  other  a  north 
pole.  If  it  be  suspended  from  the  middle  by  a  thread  it  will  point  north 
and  south.  A  knife- blade  may  be  magnetized  by  rubbing  it  several  times, 
in  the  same  direction,  across  one  of  the  poles  of  the  magnet. 


286 


MAGNETISM, 
FIG.  196. 


The  Compass. 

field.  The  directions  which  these  bodies  tend  to 
assume  are  called  lines  of  force.  Over  a  bar  magnet 
lay  a  sheet  of  paper  or  a  plate  of  glass,  and  sprinkle 
fine  iron  filings  over  this.  On  gently  tapping  the 


PIG.  197. 


POLABITT  OF  THE  NEEDLE. 


287 


FIG.  198. 


Pio. 


plate  they  become  arranged  in  curved  lines,  many  of 
which  seem  to  radiate  from  the  poles.  These  are 
the  indicators  of  the  lines  of  force,  the  position  of 
each  iron  filing  being  determined  by  its  direction 
and  distance  from  the  two  poles  of  the  magnet. 

Between  the  two  opposite  poles  of  a  horseshoe 
magnet,  or  of  two  separate  magnets  brought  near 
together,  the  lines  of  force  are  straight. 

8.  Polarity  of  the  Needle. — WHY  THE  NEEDLE 
POINTS  NORTH  AND  SOUTH.— The  earth  is  a  great  mag- 
net, whose  opposite  poles  produce  lines  of  force  that 
permeate  its  body  and  the  space  around.  A  needle 
when  magnetized  tends  to  assume  the  direction  of 
the  line  of  force  that 
passes  through  it.  The 
position  of  the  terres- 
trial magnetic  poles  is 
not  constant,  and  hence 
the  needle  changes  its 
direction  accordingly. 

Suppose  a  magnet  g 
NS  passing  through  the 
center  of  a  small  globe. 
The  needle  sn  will  hang  parallel  to  it  (Fig.  198),  its 
positive  pole  being  attracted  by  the  negative  pole  of 
the  magnet,  and  vice  versa.  If  the  globe  be  revolved 
(Fig.  199),  the  positive  pole  of  the  needle  will  turn — 
dip,  as  it  is  termed — downward.  If  the  globe  be  re- 
volved in  the  other  direction,  the  negative  pole  of  the 
needle  will  dip  in  the  same  manner.* 

*  Similar  phenomena  are  noticed  in  the  compass.     At  the  magnetic 


Magnet  in  Globe. 


288  MAGNETISM. 

A  DIPPING-NEEDLE  is  poised  as  shown  in  Fig.  200. 
At  the   magnetic   equator  it  hangs  horizontally,  but 
FIG  200  when   carried  north  its  positive   end 

dips  downward  more  and  more  until 
it    points    vertically    downward    at    a 
point  in  Boothia  Peninsula,  north  of 
Hudson   Bay,  called   the  pole   of  ver- 
ticity,  or  often  simply  the   magnetic 
pole.    This  is  the  negative  pole  of  the 
The  Dipping-needle,    earth.      The    position    of    its    positive 
pole  has  been  calculated  to  be  beneath  the  Antarctic 
Ocean.* 

9.  Magnetism  of  the  Earth. — Magnetic  bodies  such 
as  iron  fences,  lightning-rods,  iron  standards  of  chairs, 
etc.,  are  found  to  be  fully  magnetic  when  examined 
with  a  magnetic  needle.  This  is  due  to  the  inductive 
action  of  the  earth's  magnetism.  In  the  northern 
hemisphere  the  upper  end  of  such  bodies  is  south- 
magnetic,  the  lower  end  north-magnetic.  In  the 
southern  hemisphere  the  polarity  is  the  reverse  of  this. 
The  cause  of  the  earth's  magnetism  and  of  the 
variations  in  it  is  not  yet  known. 

equator  it  is  horizontal,  but  dips  whenever  taken  north  or  south.  An  un- 
magnetized  needle,  if  poised,  in  our  latitude,  on  being  magnetized,  settles 
down,  as  if  the  north  end  were  the  heavier.  This  is  remedied  by  making 
the  north  end  of  the  needle  lighter,  or  by  attaching  a  little  weight  upon 
the  south  end.  The  reverse  is  true  in  the  southern  hemisphere. 

*  The  declination  and  dip  of  the  magnetic  needle  have  daily  and  yearly 
variation,  and  also  very  slow  changes  requiring  centuries  to  complete.  In 
1686,  at  New  York,  the  declination  was  9°  west;  in  1750,  6°  20'  west;  In 
1790,  4°  15'  west;  in  1847,  6°  30'  west;  in  1885,  8°  west.  The  line  of  no 
variation  was  becoming  slowly  shifted  eastward  from  1686  to  1790,  then 
became  stationary,  and  has  since  been  moving  westward.  The  intensity  of 
terrestrial  magnetism  at  any  given  place  has  also  daily  variations,  growing 
stronger  by  day  and  weaker  by  night. 


HISTORICAL     SKETCH.  289 


SUMMARY. 

NATURAL  magnets  are  found  in  certain  regions,  but  in 
practice,  magnets  made  of  steel  are  generally  used.  These 
bars  may  be  magnetized  either  by  contact  with  other  magnets 
or  by  placing  them  within  the  magnetic  field  of  a  coil  conduct- 
ing an  electric  current  (see  p.  332).  The  existence  of  magnetism 
is  manifested  by  polarity.  Like  poles  repel,  unlike  attract  each 
other.  The  intensity  of  the  force  varies  inversely  as  the  square 
of  the  distance.  A  magnet  induces  magnetism  in  any  neighbor- 
ing magnetic  body.  This  is  not  prevented  by  intervening  bodies 
which  are  not  themselves  magnetic.  If  free  to  move,  small 
bodies  thus  influenced  by  induction  tend  to  place  themselves  in 
certain  directions,  called  lines  of  force,  around  the  inducing 
magnet.  The  declination,  the  dip,  and  the  intensity  are  the 
magnetic  elements  of  a  place.  Each  of  these  is  subject  to  daily 
variations,  and  additionally  to  slow  changes  requiring  many 
years  for  a  cycle.  The  cause  of  the  earth's  magnetism  is  un- 
known. Sudden  variations  in  it  accompany  the  outbreak  of 
spots  on  the  sun,  and  magnetic  storms  are  usually  attended  by 
the  appearance  of  the  aurora. 


HISTORICAL     SKETCH. 

MAGNETS  probably  became  first  known  to  European  nations 
through  the  discovery  of  natural  magnets  by  the  Greeks  in  the 
Thessalian  district  of  Magnesia.  From  this  the  name  was 
taken.  The  tendency  of  a  magnetized  needle  to  point  in  a  defi- 
nite direction  was  early  noticed,  and  it  is  thought  that  the 
compass  was  invented  by  the  Chinese.  The  first  mention  of  the 
use  of  the  magnetic  needle  in  Europe  occurs  in  1190.  The 
needle  was  floated  on  a  cork,  and  in  this  way  it  served  as  a 
guide  to  the  Chinese  travelers.  By  the  end  of  the  fifteenth  cent- 
ury the  compass  was  known  to  most  European  sailors,  and  its  use 
was  specially  frequent  among  the  Spanish  and  Portuguese.  The 
declination  of  the  needle  was  known  to  the  Chinese  in  the  be- 
ginning of  the  twelfth  century.  Columbus  discovered  it  inde- 


290  MAGNETISM. 

pendently  in  1482,  just  ten  years  before  his  discovery  of  Amer- 
ica. The  first  known  work  for  the  use  of  seamen  was  written 
during  the  reign  of  Queen  Elizabeth.  It  was  entitled  "A  Dis- 
course on  the  Variation  of  the  Cumpas  or  Magneticall  Needle," 
and  is  dedicated  to  "the  travaillers,  sea-men,  and  mariners  of 
England."  The  dip  was  discovered  accidentally  in  1576  by  Robert 
Norman,  an  English  instrument-maker.  He  found  the  dip  at 
London  to  be  71°  50'.  Dr.  Gilbert,  the  physician  of  Queen  Eliza- 
beth, published  his  great  work,  "De  Magnete,"  about  1600.  In 
this  he  announces  his  belief  that  the  earth  is  a  great  magnet, 
controlling  the  direction  of  the  needle.  The  variation  in  intensity 
of  the  earth's  magnetic  force  has  become  known  chiefly  during 
the  present  century. 


ELECTRICITY. 


"  MAXWELL  has  demonstrated  that  luminous  vibrations  can  be  nothing 
else  than  periodic  variations  of  electro- magnetic  forces.  Hertz,  in  proving 
by  experiments  that  electro-magnetic  oscillations  are  propagated  like  light, 
has  given  an  experimental  basis  to  the  theory  of  Maxwell.  This  gave  birth 
to  the  idea  that  the  luminiferous  ether  and  the  seat  of  electric  and  magnetic 
forces  are  one  and  the  same  thing.  This  being  established  I  can  now  .  .  . 
reply  to  the  question  you  put  to  me  :  What  is  electricity  ?  It  is  not  only  the 
formidable  agent  which  now  and  then  shatters  and  tears  the  atmosphere, 
terrifying  you  with  the  crash  of  its  thunder,  but  it  is  also  the  life-giving 
agent  which  sends  from  heaven  to  earth,  with  light  and  heat,  the  magic  of 
colors  and  the  breath  of  life.  It  is  that  which  makes  your  heart  beat  to  the 
palpitations  of  the  outside  world ;  it  is  that  which  has  the  power  to  transmit 
to  your  soul  the  enchantment  of  a  look  and  the  grace  of  a  smile."— PBOF. 
FERRARIS. 


ANALYSIS  OF  ELECTRICITY. 


I.  FBICTIONAL  ELEOTBICITY. 


H.  VOLTAIC  ELEOTBICITY. 


IH.  TBANSFOBMATIONS    or 
ELECTBIO  ENEBGY. 


1.  Development  of  Electricity. 

2.  The  Electroscope. 

3.  Electrical  Attraction  and  Repulsion, 

4.  Theory  of  Electricity. 

5.  Electrical  Potential. 

6.  Electrical  Conduction. 

7.  Electrical  Density. 

8.  Electrical  Induction. 

9.  The  Plate  Machine. 

10.  Theory  of  Attraction. 

11.  Free  and  Bound  Charges. 

12.  Inductive  Capacity. 

13.  Electrical  Condensation. 

14.  The  Leyden  Jar. 

15.  The  Voss  Machine. 

16.  Lightning. 

17.  Effects  of  Fractional  Electricity. 

1.  Simple  Voltaic  Cell. 

2.  Action  in  the  Voltaic  Cell. 

3.  The  Electric  Current. 

4.  The  Volt. 

5.  Electrical  Resistance. 

6.  The  Ohm. 

7.  The  Ampere. 

8.  The  Battery. 

9.  Polarization  within  the  Battery. 

10.  Special  Forms  of  Battery. 

11.  Effects  of  Voltaic  Electricity. 

1.  Effect  of  a  Voltaic  Current  on  a  Mag- 

netic Needle. 

2.  Magnetic  Coils. 

3.  The  Tangent  Galvanometer. 

4.  The  Electro-magnet. 

5.  The  Electro-magnetic  Telegraph. 

6.  Ocean  Cables. 

7.  Current  Induction. 

8.  The  Induction  Coil. 

9.  Magneto-induction. 

10.  The  Telephone. 

11.  The  Microphone. 

12.  The  Magneto-electric  Machine. 

13.  The  Dynamo-electric  Machine. 

14.  The  Electric  Light. 

15.  Thermo-electricity. 

16.  Animal  Electricity. 


ELECTRICITY. 

a  piece  of  sealing-wax  is  rubbed  with  flannel 
it  temporarily  acquires  the  new  property  of  attracting, 
when  brought  near  them,  other  light  bodies,  such  as 
shreds  of  paper,  gold  leaf,  lint,  etc.  If  .the  hand  is  ap- 
proached sufficiently  near  the  excited  sealing-wax,  small 
sparks  may  not  infrequently  be  seen  in  the  dark  to  leap 
across  the  space  between  the  hand  and  the  sealing- 
wax.*  The  excited  sealing-wax  is  evidently  in  a  dif- 
ferent state  from  what  it  was  before  it  was  rubbed 
with  the  flannel.  The  supposed  agent  producing  this 
state  is  called  electricity,  and  the  sealing-wax  in  this 
excited  condition  is  said  to  be  electrified.  Other  sub- 
stances may  be  used  instead  of  the  sealing-wax.  A 
piece  of  vulcanite,  glass,  resin,  shellac,  or  amber  will  do 
as  well.  The  substance  also  with  which  friction  is  pro- 
duced may  be  varied.  Instead  of  flannel,  fur,  silk,  or 
chamois-skin  covered  with  tin-amalgam  may  be  used, 
and  it  is  found  that  with  some  rubbers  better  results  are 
.obtained  than  with  others.  Under  proper  conditions 
most  bodies  may  be  rendered  electric  by  friction. 

*  In  cold,  frosty  weather,  a  person,  by  shuffling  about  in  his  stocking- 
feet  upon  the  carpet,  can  develop  so  much  electricity  in  his  body  that  he 
can  ignite  a  jet  of  gas  by  simply  applying  his  finger  to  it.— Blasts  in  mines 
intended  to  be  fired  by  electricity  have  thus  been  prematurely  discharged 
by  the  workmen  touching  the  wires.  To  prevent  this  disastrous  effect,  at 
tne  Sutro  Tunnel,  Nevada  City,  the  workmen  who  are  handling  exploders 
wet  their  boots,  stand  on  an  iron  plate  to  conduct  off  the  electricity  of  the 
body,  and  wear  rubber  gloves. 


294 


ELECTRICITY. 


I.    FRICTIONAL  ELECTRICITY. 

1.  Electricity  developed  either  directly  or  indirectly 
by  friction  is  called  frictional  electricity.  For  its  detec- 
tion it  is  convenient  to  employ  instruments  specially 
adapted  for  this  purpose,  called  electroscopes. 

2.  The  Electroscope. — A  pith  ball  suspended  by  a 
silk  thread,  as  shown  in  Fig.  201,  constitutes  a  simple 
and  very  effective  electroscope.    A  straw  3  or  4  inches 


Electroscopes. 


in  length,  and  suspended  by  a  silk  fibre  at  the  middle 
so  as  to  hang  horizontally,  may  be  substituted  for  the 
pith  ball.  A  lath  balanced  on  an  egg  placed  in  a  wine- 
glass may  also  serve  as  an  electroscope. 

3.  Electrical  Attraction  and  Repulsion. —  If  a  warm 
dry  glass,  such  as  a  lamp-chimney,  be  rubbed  with  a 
silk  handkerchief,  a  crackling  sound  will  be  heard. 


FRICTIONAL    ELECTRICITY.  295 

If  the  tube  be  held  near  the  face,  a  sensation  like 
that  of  touching  cobwebs  will  be  felt.  Make  with  the 
electrified  tube  the  following  experiments.*  Present 


*  The  following  simple  experiments  are  instructive :— 1.  A  rubber  comb 
passed  a  few  times  through  the  hair  will  furnish  enough  electricity  to  turn 
the  lath  entirely  around,  and  empty  egg-shells,  paper  hoops,  etc.,  will  follow 
the  comb  over  the  table  in  the  liveliest  way.— 2.  Take  a  thin  sheet  of  gutta- 
percha,  about  a  foot  square ;  lay  it  upon  the  table,  and  rub  it  briskly  a  few 
times  with  an  old  fur  cuff ;  the  gutta-percha  will  become  powerfully  elec- 
trified.—3.  Lift  the  gutta-percha  by  one  corner,  and  some  force  will  be  re- 
quired to  separate  it  from  the  table.— 4.  Hold  the  electrified  gutta-percha  in 
the  left  hand ;  bring  the  fingers  of  the  right  near  the  paper ;  it  will  be  at- 
tracted to  the  hand,  and  sparks  will  pass  to  the  fingers  with  a  snapping 
sound.— 5.  Hold  some  feathers,  suspended  by  a  silk  thread,  near  the  excited 
gutta-percha,  and  the  feathers  will  be  attracted.— 6.  Hold  the  excited  paper, 
or  the  excited  sheet  of  gutta-percha,  over  the  head  of  a  person  with  dry 
hair;  the  hair  will  be  attracted  by  the  gutta-percha,  and  each  particular 
hair  will  stand  on  end. — 7.  Hold  the  excited  gutta-percha  near  the  wall ; 
the  gutta-percha  will  fly  to  it,  and  remain  some  minutes  without  falling. — 
8.  Place  a  sheet  of  gutta-percha  on  a  tea-tray ;  rub  the  gutta-percha  briskly 
with  a  fur  cuff ;  place  the  tea-tray  with  the  excited  sheet  of  gutta-percha 
on  a  dry  tumbler ;  lift  off  the  gutta-percha  from  the  tea-tray ;  bring  the 
knuckle  of  your  hand  near  the  tray,  and  you  will  receive  a  spark.  Replace 
the  gutta-percha  on  the  tray  and  apply  your  knuckle,  and  you  will  receive 
another  spark.  This  may  be  repeated  a  dozen  times,— 9.  Take  a  sheet  of 
foolscap  paper  and  a  board  about  the  same  size.  Heat  both  till  they  are 
thoroughly  dry.  While  hot,  lay  the  paper  on  the  board  and  rub  the  former 
briskly  with  a  piece  of  rubber.  The  paper  and  board  will  cling  together. 
Tear  the  paper  loose  and  try  experiments  4,  5,  6,  and  7.  Return  the  paper 
and  rub  as  before.  Cut  the  paper  so  as  to  form  a  tassel.  Then  lift,  and  the 
strips  of  the  tassel  will  repel  one  another.— 10.  Take  a  piece  of  common 
brown  paper,  about  the  size  of  an  octavo  book,  hold  it  before  the  fire  till 
quite  dry  and  hot,  then  draw  it  briskly  under  the  arm  several  times,  so  as 
to  rub  it  on  both  sides  at  once  by  the  coat.  The  paper  will  be  found  so 
powerfully  electrical,  that  if  placed  against  a  wainscoted  or  papered  wall  of 
a  room,  it  will  remain  there  for  some  minutes  without  falling. — 11.  While 
the  paper  still  clings  to  the  wall  hold  against  it  a  light,  fleecy  feather,  and  it 
will  be  attracted  to  the  paper  in  the  same  way  the  paper  is  to  the  wall.— 
12.  If  the  paper  be  warmed,  drawn  under  the  arm  as  before,  and  then 
hung  up  by  a  thread  attached  to  one  corner,  it  will  sustain  several  feathers 
on  each  side ;  should  these  fall  off  from  different  sides  at  the  same  time, 
they  will  cling  together  very  strongly;  and  if  after  a  minute  they  are  all 
shaken  off,  they  will  fly  to  one  another  in  a  singular  manner.— 13.  Warm 
and  excite  the  paper  as  before,  and  then  lay  on  it  a  ball  of  elder-pith, 


296  ELECTRICITY. 

it  to  the  pith  ball  of  an  electroscrope.  This  will  be 
attracted  till  it  touches,  and  then  fly  off.  The  end 
of  the  suspended  straw  will  likewise  be  first  attracted, 
but  then  repelled  just  after  it  is  touched.  Grasp  the 
pith  ball  or  straw  for  a  moment.  It  will  no  longer 
be  repelled.  Rub  a  stick  of  sealing-wax  with  a  woolen 
cloth  or  some  fur.  The  behavior  of  the  pith  ball  or 
straw  toward  it  will  be  the  same  as  toward  the  glass. 
But  bring  the  rubbed  sealing-wax  near  to  the  pith 
ball  or  straw  that  is  repelled  by  the  rubbed  glass ; 
there  will  be  attraction  instead  of  repulsion.  If  the 
excited  glass  be  held  on  one  side  of  a  ball  and  the 

about  the  size  of  a  pea ;  the  ball  will  immediately  roll  across  the  paper, 
and  if  a  needle  be  pointed  toward  it,  it  will  again  roll  to  another  part,  and 
so  on  for  a  considerable  time.— 14.  Support  a  pane  of  glass,  well  dried  and 
warmed,  upon  two  books,  one  at  each  end,  and  place  some  bran  underneath ; 
then  rub  the  upper  side  of  the  glass  with  a  silk  handkerchief,  or  a  piece  of 
flannel,  and  the  bran  will  dance  up  and  down  like  the  images  in  Fig.  208. 
—15.  Place  a  common  tea-tray  on  a  dry,  clean  tumbler.  Then  take  a  sheet 
of  foolscap  writing-paper  (as  in  No.  9)  and  dry  it  carefully  until  all  its 
hygrometric  moisture  is  expelled.  Holding  one  end  of  the  sheet  on  a  table 
with  the  finger  and  thumb,  rub  the  paper  with  a  large  piece  of  India  rubber 
a  dozen  times  vigorously  from  left  to  right,  beginning  at  the  top.  Now  take 
up  the  sheet  by  two  of  the  corners  and  bring  it  over  the  tray,  and  it  will 
fall  like  a  stone.  This,  as  well  as  the  apparatus  in  No.  8,  forms  a  simple 
Electroplvorus,  fit  to  perform  many  experiments  ordinarily  performed  with 
that  instrument.  If  the  tip  of  a  finger  be  held  close  to  the  bottom  of  the 
tray,  a  sensible  shock  will  be  felt.  Next,  lay  a  needle  on  the  tray  with  its 
point  projecting  outward,  remove  the  paper,  and,  in  the  dark,  a  star  sign 
of  the  negative  electricity  will  be  seen ;  return  the  paper,  and  the  positive 
brush  will  appear.  Lay  a  dry,  hot  board,  as  in  No.  9,  on  top  of  four  tum- 
blers. If  a  boy  stand  on  the  board  he  will  be  insulated,  and  on  his  holding 
the  tray  vertically,  the  paper  will  not  fall.  Sparks  may  then  be  drawn 
from  his  body,  and  his  hair  will  be  electrified.— 16.  Warm  a  lamp-chimney, 
rub  it  with  a  hot  flannel,  and  then  bring  a  downy  feather  near  it.  On  the 
first  moment  of  contact,  the  feather  will  adhere  to  the  glass,  but  soon  after 
will  fly  rapidly  away,  and  you  may  drive  it  about  the  room  by  holding  the 
glass  between  it  and  the  surrounding  objects ;  should  it,  however,  come  in 
contact  with  any  thing  not  under  the  influence  of  electricity,  it  will  in- 
stantly fly  back  to  the  glass. 


FRICTIONAL     ELECTRICITY.  297 

excited  wax  on  the  other,  it  will  fly  between  the  two, 
touching  each  in  succession  alternately.  From  this 
we  conclude  that  (1),  there  are  two  kinds  of  mani- 
festation of  frictional  electricity ;  and  (2),  like  kinds 
are  manifested  ~by  repulsion,  and  unlike  ~by  attrac- 
tion. The  electricity  from  the  glass  is  termed  posi- 
tive [  +  ],  and  that  from  the  wax,  negative  [— ].* 

4.  Theory  of  Electricity.— It  is  thought  that  posi- 
tive and  negative  electricity  exist  in  every  body,  in 
a  state  of  total  or  partial  equilibrium.    When  this  is 
disturbed,  as  by  friction,  electrical  separation  follows, 
and  each  kind  becomes  manifested,  just  as  in  the 
polarization  of  a  magnet,  if  the  proper  conditions  are 
observed.     Electricity    is    not    a    fluid,   as    was   long 
taught.    It  may  be  a  condition  of  strain  among  the 
molecules  of  a  body,  capable  of  being  communicated 
like  a   fluid.     We   know  only   its   laws,  and   not   its 
nature. 

5.  Electric  Potential. — A  body  electrically  excited 
by  friction  or  otherwise  is  said  to  be  charged.     The 
charge  may  be  either  positive  or  negative,  strong  or 
weak.    If  two  bodies  equally  and  oppositely  charged 
are  put  into  contact,  the  charge  of  each  is  neutral- 
ized by  that  of  the  other.     A  body  strongly  charged 
positively  is  said  to  be  at  high  potential;    if  nega- 

*  In  the  following  list,  each  substance  becomes  positively  electrified 
when  rubbed  with  the  body  following  it ;  but  negatively,  with  the  one  pre- 
ceding it.— GANOT. 

1.  Cat's  fur.          5.  Cotton.  9.  Shellac.  13.  Caoutchouc. 

2.  Flannel.  6.  Silk.  10.  Resin.  14.  Gutta-percha. 

3.  Ivory.  7.  The  hand.        11.  The  metals.  15.  Gun-cotton. 

4.  Glass.  8.  Wood.  12.  Sulphur. 


298  ELECTRICITY. 

tively,  at  low  potential;  when  discharged,  at  zero 
potential  The  surface  of  the  earth  is  electrically 
at  zero. 

6.  Conductors    and    Insulators.  —  A   body  which 
allows  electricity  to  pass  through  it  freely  is  termed 
a   conductor;   one   which   does   not,   is  called   a   ~bad 
conductor,  or  insulator.     Copper  is  one  of  the  best 
conductors,  and  hence  it  is  used  in  many  electrical 
experiments.     Glass   is   one   of   the   best   insulators. 
A  body  is  said  to  be  insulated  when  it  is  supported 
by  some  bad  conductor,  which  is  generally  glass  or 
vulcanite.    A  body  can  be  highly  charged  only  when 
insulated.     In   damp  air   electricity  is   quickly  dissi- 
pated.   This  is  due  to  the  deposit,  on  the  glass  insu- 
lators,  of  a  thin  film  of   moisture,  which    conducts 
away  the  electricity.    For  success  in  electrical  experi- 
ments, therefore,  it  is  important  to  keep  the  air  dry 
and  warm,  since  dry  air  is  one  of  the  best  of  insu- 
lators.* 

7.  Distribution  of  Electricity  on  Bodies. — A  charge 
communicated    to   one    part   of   an   insulator  is   not 
spread  over  its  whole  surface ;  but  when  a  good  con- 
ductor is  charged  at  any  point  the  spread  is  instan- 
taneous.   It  spreads,  however,  only  on  the  surface,  and 

*  The  following  list  contains  some  of  the  most  common  conductors  and 
insulators : 

Conductors.  Insulators. 

Metals.                Vegetables.  Dry  Air.             GMass. 

Charcoal.            Animals.  Shellac.                Silk. 

Flame.                 Linen.  Amber.                Dry  Paper. 

Minerals.             Cotton.  Sulphur.              Caoutchouc. 

Acids.                  Dry  Wood.  Wax. 
Water.                Ice. 


FKICTIONAL     ELECTRICITY. 


299 


not  through   the   interior.     A  pith  ball,  if  made  to 
touch  the  outside  of  an  electrified  metal  cup  or  hol- 


Fra.  202. 


Variation  in  Electric  Density. 

low  ball,  is  strongly  repelled ;    but  on  the   interior 
there  is  no  such  effect.*    If  the  ball  is  spherical,  the 


FIG.  203. 


*  Faraday  once  made  a  hollow  cube  of  wood,  measuring  12  ft.  each  way 
and  covered  with  tin-foil.  Insulating  this,  he 
charged  it  with  a  powerful  machine  until 
sparks  darted  off  from  every  corner  on  the 
outside.  Going  within  this  little  room  with 
his  most  delicate  electroscopes,  he  could  not 
detect  the  least  effect  upon  them.  He  made 
a  conical  bag  of  linen,  and  fastened  its  open 
end  to  an  insulated  ring.  Pulling  it  out  with 
a  silken  cord,  he  electrified  it.  The  charge 
was  manifest  on  the  outside,  zero  on  the 
inside.  Beversing  the  pull  so  as  to  turn  it 
inside  out,  the  new  exterior  was  found  to  be 
charged.  A  half -minute  previously  it  had 
been  a  neutral  interior.  The  student  should 
try  this  interesting  experiment,  using  the 
most  delicate  electroscope  that  he  can  make.  Faraday's  Conical  Bag. 


300 


ELECTRICITY. 


Fio.  204. 


amount  of  electricity  at  all  points  of  its  surface  is 
the  same ;  or,  we  may  say  that  the  electric  density 
is  uniform  over  its  surface.  On  a  cylinder  the  elec- 
tric density  is  greatest  at  the  ends.  If  one  end  is 
blunt  and  the  other  sharp,  the  density  at  the  sharp 
end  becomes  so  great  that  the  neighboring  air  mole- 
cules are  quickly  electrified  by  contact  and  instantly 
repelled.  Others  in  turn  are  successively  repelled, 
and  the  body  is  soon  discharged.  Electricity  thus 
escapes  rapidly  from  jutting  points.* 

8.  Electrical  Induction.  —  Let  an  insulated  con- 
ductor, Fig.  204,  be  brought  near  another  conductor 

that  has  been  strongly 
Sk  charged  positively,  and 
let  a  series  of  pairs  of 
pith  balls  be  suspend- 
ed from  the  first.  The 
motion  of  the  balls 
shows  that  the  ends 
of  the  insulated  con- 
ductor are  electrically 
excited,  while  the  mid- 
dle is  neutral.  The  end 
Electrical  induction.  nearest  the.  charged 

conductor  is  excited  negatively  and  the  remote  end 
positively.  If  the  charged  conductor  be  removed,  all 
of  the  pith  balls  collapse.  Place  several  insulated 

*  The  electric  whirl,  mounted  on  the  prime  conductor  of  an  electrical 
machine,  illustrates  this  action.  As  each  molecule  of  air  is  repelled  from 
a  point,  it  reacts  with  equal  force  against  the  point.  This  is  sufficient  to 
set  the  light  wire-wheel  in  rapid  rotation. 


FRICTIONAL    ELECTRICITY.  301 

conductors,  as  shown  in  Fig.  205,  the  balls  being 
strongly  charged,  that  at  the  right  positively,  and 
that  at  the  left  negatively.  Each  intermediate  con- 
ductor becomes  excited,  as  indicated,  and  becomes 

FIG.  205. 


Electrical  Induction. 

neutral  when  the  balls  are  discharged.  It  has  been 
polarized  by  induction,  like  a  magnetic  body  when 
brought  into  the  field  of  a  magnet  pole.* 

9.  The  Plate  Electrical  Machine  consists  of  (1) 
a  circular  glass  plate  which  can  be  turned  by  means 
of  a  crank ;  (2)  a  pair  of  leather  or  cloth  rubbers 
pressed  against  the  plate  and  covered  with  electrical 
amalgam  or  tin  disulphide ;  f  (3)  a  metallic  comb  or 
fork  with  sharp  points  which  nearly  touch  the  plate ; 
(4)  a  prime  conductor,  consisting  of  a  rounded  brass 
cylinder,  insulated  by  resting  on  a  glass  standard, 
and  connected  at  one  end  with  the  comb.  Frequently 

*  The  experiment  in  Fig.  204  can  be  nicely  performed  by  means  of  an 
egg  placed  flatwise  on  the  top  of  a  dry  wine-glass  and  the  glass  tube  rep- 
resented in  Pig.  201.  Several  eggs  and  glasses  will  show  the  principle  of 
Fig.  205.  See  TyndalFs  "  Lessons  in  Electricity,"  p.  39. 

t  Electrical  amalgam  is  a  mixture  of  two  parts  each  of  tin  and  zinc,  and 
four  parts  of  mercury.  By  experience  it  has  been  found  that,  when  this  ia 
rubbed  on  glass,  electrical  separation  is  most  easily  effected.  Tin  disulphide 
is  often  called  "mosaic  gold,"  because  of  its  metallic  yellow  color.  It  is 
used  in  bronzing. 


302 


ELECTRICITY. 


the  lower  half  of  the  plate  is  made  to  revolve  between 
a  pair  of  silken  flaps  (Fig.  206).  A  chain  is  usually 
attached  to  the  knob  in  connection  with  the  rubber, 
and  connects  this  with  the  ground  through  the  me- 
dium of  a  gas-pipe  or  other  conductor. 

On  turning  the   crank,  the   friction   of  the  plate 
against  the   rubbers  produces   electrical   separation ; 


FIG.  206. 


The  Plate  Electrical  Machine. 

the  rubbers  becoming  charged  negatively,  the  plate 
positively.  The  negative  charge  is  conducted  off  to 
the  earth  by  the  chain,  which  thus  restores  the  rub- 
bers to  zero  potential.  The  positive  charge  on  the 
plate,  when  this  is  brought  opposite  the  comb,  polar- 
izes the  prime  conductor  and  comb  by  induction, 
Positive  electricity  becomes  manifested  on  the  re- 
mote conductor,  and  negative  electricity  at  the  comb 


FKICTIONAL     ELECTRICITY. 


303 


is  communicated  at  once  by  the  sharp  points  to 
the  air,  whose  molecules  are  repelled  into  contact 
with  the  plate,  thus  neutralizing  its  positive  charge. 
The  prime  conductor  is  hence  left  charged  to  high 
potential. 

The  action  of  the  plate  machine  is  thus  an  appli- 
cation of  both  friction  and  induction. 

1C.  The  Theory  of  Attraction  is  likewise  an  ap- 
plication of  induction.  In  Fig.  201,  where  a  glass 
rod  at  high  potential  is  brought  near  a  pith  ball, 
this  is  polarized  by  induction,  the  nearer  half  becom- 
ing negative,  and  the  remote  half  positive.  The 
charge  on  the  rod  attracts  the  negative  half  and  re- 
pels the  positive  half.  But  since  the  negative  half 
is  nearer,  the  attraction  exceeds  the  repulsion,  and 
the  pith  ball  moves  toward  the  rod.  On  touching 
this  the  negative  charge  is  wholly  neutralized,  and 
only  repulsion  can  be  effective.  Every  case  of  elec- 
trical attraction  is  thus  a  case  of  induction. 

The  electric  chime  consists  of  three  bells,  two 
of  which,  c  and  5,  are  hung 
by  brass  chains,  while  the 
middle  one  is  insulated  above 
by  a  silk  cord,  and  connected 
below  with  the  earth  by  a 
chain.  The  balls  between 
them  are  also  insulated.  The 
outer  bells  becoming  charged 
with  positive  electricity  from 
the  prime  conductor  of  an  Biectnc  chimes, 

electrical  machine,  polarize   the  balls   by  induction 


FIG.  207. 


304 


ELECTRICITY. 


FIG.  208. 


through  the  intervening  air.     The  balls  being  then 
attracted  to  the  bells,  are  charged  and  immediately 

repelled.  Swinging  away,  they 
strike  against  the  middle  bell, 
discharging  their  electricity, 
and  are  forthwith  attracted 
again.  Flying  to  and  fro,  they 
ring  out  a  merry  song. 

The  dancing  image  consists 
of  a  pith-ball  figure  placed  be- 
tween two  metallic  plates,  the 
upper  one  hanging  from  the 

Dancing  Images.  prime   conductorj  and   the  lower 

one   connected  with  the  earth.     The   dance  is   con- 
ducted by  alternate  attraction  and  repulsion.* 

11.  Free  and  Bound  Electricity.— The  gold-leaf 
electroscope  is  more  sensitive  than  one  of  pith  balls. 
Within  a  dry  glass  jar  a  pair  of  strips  of  gold-leaf 
are  suspended  from  a  metal  rod  terminating  at  the 
top  in  a  knob  or  plate.  If  a  rod,  excited  for  example 
negatively,  be  brought  near  the  knob,  then  by  induc- 
tion this  becomes  charged  positively  while  both 
leaves  are  charged  negatively,  and  hence  repel  each 
other.  By  placing  the  finger  on  the  knob  and  with- 
drawing it  while  the  rod  is  still  near,  the  leaves  col- 
lapse. Their  negative  charge  has  been  conducted  off 
to  the  earth.  But  on  withdrawing  now  the  rod,  they 
diverge  again  and  remain  apart.  The  positive  charge 

*  A  slow  motion  should  be  given  to  the  electrical  wheel,  and  a  pin 
thrust  into  the  heel  of  the  image  will  add  much  to  the  stamp  of  the  tiny 
feet. 


FKICTIONAL     ELECTRICITY. 


305 


on  the  knob  was  "bound"  there  by  the  presence  of 
the  negatively  excited  rod  and* could  not  be  con- 
ducted away,  like 
the  negative  charge 
on  the  leaves.  On 
removing  the  rod 
after  the  finger  has 
been  taken  away  the 
positive  charge  be- 
comes "free  "  ;  it  is 
distributed  over 
knob  and  leaves, 
and  these  now  repel 
each  other  with  a 
positive  charge.  So 
long  as  a  charge  is 
"  bound,"  i.  e.,  so  long 
as  the  knob  and 
leaves  under  the  in- 

fluence  of  the  excited  rod  are  at  zero  potential,  it  fails 
to  manifest  itself. 

12.  Inductive  Capacity. —  A  body  through  which 
induction  occurs  is  called  a  dielectric.   The  power  with 
which  inductive  influence  acts  across  a  dielectric  is 
called  its  inductive  capacity,  and  this  is  different  for 
different    dielectrics.      Induction    takes    place   better 
through  a  plate  of  glass  than  through  a  layer  of  equal 
thickness  of  shellac  or  air.    In  other  words,  glass  has  a 
higher  inductive  capacity  than  shellac  or  air,  and  is, 
therefore,  the  best  dielectric  of  the  three. 

13.  Electrical  Condensation.—  By  putting  a  good 


306  ELECTRICITY. 

dielectric  between  two  conducting  surfaces,  one  of 
which  is  connected  with  the  prime  conductor  of  an 
electrical  machine  and  the  other  with  the  earth,  elec- 
tricity may  be  strongly  " condensed"  on  these  sur- 
faces. In  Fig.  210,  let  the  strip  on  the  left  represent 
a  conductor,  of  tin-foil,  positively  charged  from  the 
machine,  and  that  on  the  right  a  similar  conductor 
connected  with  the  earth,  the  intervening  space  being 
occupied  by  a  plate  of  glass.  This  dielectric  becomes 
polarized,  the  surface  on  the  right  attaining  a  nega- 
tive charge  which  is  bound  there,  while  the  corre- 
sponding positive  electricity  on  the  same  side  is  neu- 
tralized by  connection  with  the  earth. 
i  2,  The  negative  charge  in  turn  reacts 

N^^^^N       through  the  dielectric,  binding  a  pos- 
itive charge  on  the  left,  whose  energy 
thus    becomes    potential.     The    con- 
ductor can  then  receive  a  new  charge 
OOC^         from  the  machine,  and  the  process 
is  repeated  until  the  greatest  charge 
is  accumulated  that  the   condenser   can   carry.     Its 
molecules  are  then  in  a  condition  of  great  strain. 

14.  The  Leyden  Jar  consists  of  a  glass  jar,  serv- 
ing as  dielectric,  coated  inside  and  outside,  not  quite 
to  the  top,  with  tin-foil.  It  is  fitted  with  a  cover  of 
baked  wood  through  which  passes  a  metal  rod  with 
a  knob  at  the  top,  and  below  a  metal  chain  extend- 
ing down  to  the  inner  coating.  The  jar  is  charged 
by  bringing  the  knob  near  the  prime  conductor  of 
the  machine,  while  the  outer  coating  communicates 
with  the  earth.  The  inner  coating  becomes  charged 


FRICTIONAL     ELECTRICITY. 


307 


FIG.  211. 


The  Leyden  Jar. 


first  from  the  machine,  a  succession  of  sparks  being 
received  until  the  two  coatings  acquire  a  large  charge 
of  bound  electricity,  positive  with- 
in and  negative  without.  To  dis- 
charge it  *  one  end  of  a  conductor 
with  an  insulated  handle  is  put 
on  the  outer  coating,  while  the 
other  is  brought  near  the  knob 
above.  A  sharp  snap  and  a  brill- 
iant flash  through  the  air  an- 
nounce that  equilibrium  is  re- 
stored. Minute  particles  detached 
from  the  solid  conductors  are 
made  momentarily  white  -  hot, 
giving  brilliancy  to  the  spark,  f 

The  tin-foil  on  a  Leyden  jar  serves  only  as  a  con- 
ductor, and  not  as  an  accumulator,  of  the  charge. 
The  jar  may  be  made  with  movable  coatings.  After 
it  is  charged  these  may  be  removed.  Putting  the 
same  jar  then  into  another  set  of  coatings,  it  may 
be  discharged  in  the  usual  manner. 

*  It  is  said  that  Cuneus,  a  pupil  at  Leyden,  discovered  the  principle  of 
the  Leyden  jar  in  the  following  curious  way:  While  experimenting,  he 
held  a  bottle  of  water  to  the  prime  conductor  of  his  electrical  machine. 
Holding  the  bottle  with  one  hand,  he  happened  to  touch  the  water  with 
the  other,  when  he  received  a  shock  so  unexpected,  and  so  unlike  any  thing 
he  had  ever  felt  before,  that  he  was  filled  with  astonishment.  It  was  two 
days  before  he  recovered  from  his  fright.  A  few  days  afterward,  in  a 
letter  to  a  friend,  the  physicist  innocently  remarked,  that  he  would  not 
take  another  shock  for  the  whole  kingdom  of  France. 

t  The  incredibly  small  quantity  of  the  metal  volatized  in  this  way  is  a 
striking  proof  of  the  divisibility  of  matter.  During  some  experiments  at 
the  Philadelphia  mint  a  gold  pole  lost  in  weight  by  a  strong  spark  one 
millionth  of  a  grain ;  and  rsso^oo  of  a  grain  of  nickel  signed  its  name  in  the 
spectroscope  brilliantly.  See  "  Popular  Science  Monthly,"  May,  1877. 


308 


ELECTRICITY. 


15.  The  Toepler-Holtz  Electrical  Machine. — Many 
improvements  have  been  made  on  the  plate  electrical 
machine.  One  of  the  best  is  the  Toepler-Holtz  m  achine.* 
This  consists  of  a  fixed  glass  plate  in  front  of  which  re- 


FIG.  212. 


The  Toepler-Holtz  Machine. 

volves  a  smaller  one  provided  with  six  metallic  buttons 
(Fig.  212,  &).  On  the  rear  of  the  fixed  plate  are  two 
sheets  of  varnished  paper,  a  and  a/,  called  the  armature. 
On  each  a  strip  of  tinfoil  is  cemented,  from  which  a 
metallic  arm  extends  around  to  the  front,  ending  in 
a  rubber  of  brass  filaments,  r.  Under  this  each  but- 

*  The  "  Wimshurst  "  electrical  machine  is  the  most  recent  improvement 
of  electrical  machines  depending  on  induction.  In  these  machines  electricity 
is  generated  by  glass  discs  faced  with  metallic  sectors,  which  are  rotated  in 
opposite  directions.  These  machines  are  certain  in  their  action,  and  furnish 
large  quantities  of  electricity  at  a  high  potential. 


FRICTIONAL     ELECTRICITY.  309 

ton  passes.  A  pair  of  combs,  c  and  c',  connect  with 
adjustable  discharging  rods,  p  and  n.  Another  pair 
of  combs  and  brushes  are  attached  to  the  brass  rod, 
dd' ;  this  extends  across  in  front  of  the  plate,  which 
revolves  in  the  direction  shown  by  the  arrows. 

If  there  be  the  least  possible  difference  of  poten- 
tial between  the  two  armatures,  such  as  is  naturally 
due  to  accidental  conditions  on  their  surfaces,  it  may 
be  greatly  increased  by  revolving  the  plate.  Suppose 
the  left  armature,  a',  to  be  faintly  charged  positively, 
while  the  right  armature,  a,  is  neutral ;  then  a'  in- 
duces a  slight  negative  bound  charge  on  the  button 
in  front,  which  in  revolving  passes  under  d'.  Passing 
from  d'  to  r,  the  button  comes  opposite  a  neutral 
armature.  Its  negative  bound  charge  at  once  be- 
comes free  and  is  conducted  through  the  rubber  r  to 
the  armature  behind,  charging  it  negatively.  This 
at  once  acts  inductively  on  the  button,  causing  it  to 
acquire  a  positive  bound  charge  with  which  it  passes 
d.  This  charge  is  freed  at  r'  and  conducted  to  the 
armature  a/,  strengthening  its  positive  charge.  This 
process  continues,  both  armatures  becoming  soon 
strongly  and  oppositely  charged.  The  comb,  c',  by 
induction  from  a,  is  polarized.  It  discharges  nega- 
tive electricity,  while  the  rod,  p,  acquires  a  strong 
positive  charge.  In  like  manner  c  discharges  positive 
electricity  and  n  acquires  a  strong  negative  charge. 
A  succession  of  sparks  soon  passes  between  p  and  n, 
the  strength  of  which  is  greatly  increased  by  con- 
densation in  the  Leyden  jars,  with  which  the  dis- 
charging rods  are  connected. 


310  ELECTKICITY. 

16.  Lightning  is  only  the  discharge  of  a  Leyden 
jar  on  a  grand  scale.  If  two  clouds  with  opposite 
charges  of  electricity  come  near  together,  the  inter- 
vening air  reaches  its  limit  of  polarization,  and  a 
flash  occurs  like  that  between  the  discharging-rods 
of  an  electrical  machine.*  The  air  is  never  quite  uni- 
form in  conducting  power  at  all  places,  and  the  im- 
mense spark,  moving  along  the  line  of  least  resist- 
ance, describes  a  zigzag  course.  It  suddenly  heats 
the  air,  which  expands  and  instantly  collapses.  The 
concussion  produces  a  series  of  air-waves  from  suc- 
cessive parts  of  the  spark.  These  constitute  thunder, 
which  continues  to  roll  because  the  sound  is  reflected 
many  times  from  clouds,  and  from  masses  of  air 
which  differ  among  themselves  in  density.  Often 
the  charged  cloud  approaches  the  ground  rather 
than  another  cloud.  Discharge  takes  place,  and  ex- 
posed objects,  such  as  tall  houses  or  trees,  are  de- 
stroyed if  included  in  the  lightning's  path. 

LIGHTNING-KODS  were  invented  by  Franklin,  f  They 
are  based  on  the  principle  that  electricity  always 

*  The  air  is  constantly  electrified.  In  clear  weather  it  is  in  a  positive 
state,  but  in  foul  weather  it  changes  rapidly  from  positive  to  negative,  and 
vice  versa.  Dr.  Livingstone  tells  us  that  in  South  Africa  the  hot  wind 
which  blows  over  the  desert  is  so  highly  electrified,  that  a  bunch  of  ostrich 
feathers  held  for  a  few  seconds  against  it  becomes  as  strongly  charged  as  if 
attached  to  an  electrical  machine,  and  will  clasp  the  hand  with  a  sharp, 
crackling  sound. 

t  Franklin's  plan  was  opposed  by  many  men  of  his  day,  who  declared 
it  was  as  impious  to  ward  off  Heaven's  lightning,  "as  for  a  child  to  ward 
off  the  chastening  rod  of  its  father."  There  was  much  discussion  as  to 
whether  the  conductors  should  be  pointed  or  not.  Wilson  persuaded 
George  III.  that  the  points  were  a  republican  device  to  injure  His  Majesty, 
as  they  would  certainly  "invite"  the  lightning,  and  so  the  points  on  the 
lightning-rods  upon  Buckingham  Palace  were  changed  for  balls. 


FRICTIONAL     ELECTRICITY.  311 

seeks  the  best  conductor.  The  rod  should  be  pointed 
at  the  top  with  some  metal  which  will  not  easily 
corrode.  If  constructed  in  several  parts,  they  should 
be  securely  jointed.  The  lower  end  should  extend 
into  water,  or  else  deep  into  the  damp  ground,  be- 
yond a  possibility  of  any  drought  rendering  the 
earth  about  it  a  non-conductor,  and  be  packed  about 
with  ashes  or  charcoal.  If  the  rod  is  of  iron,  it 
needs  to  be  much  larger  than  one  of  copper,  which 
is  a  better  conductor.  Every  elevated  portion  of  the 
building  should  be  protected  by  a  separate  rod. 
Chimneys  need  especial  care,  because  of  the  ascend- 
ing column  of  vapor  and  smoke.  Water  conductors, 
tin  roofs,  etc.,  should  be  connected  with  the  damp 
ground  or  the  lightning-rod,  that  they  may  aid  in 
conveying  off  the  electricity.* 

DURATION  OF  THE  FLASH.  —  The  duration  of  the 
flash  from  a  Ley  den  jar  has  been  found  to  vary  from 
two  thousandths  to  forty  billionths  of  a  second. 
When  the  plate  of  the  Toepler-Holtz  machine  is  re- 
volving at  the  highest  speed,  each  button  can  be  mo- 
mentarily seen,  as  if  it  were  still,  when  illuminated  by 
the  spark.  The  trees  swept  by  the.  tempest,  or  a  train 
of  cars  in  rapid  motion,  when  seen  by  a  flash  of 


*  The  value  of  a  lightning-rod  consists,  most  of  all,  in  its  power  of 
quietly  restoring  the  equilibrium  between  the  earth  and  the  clouds.  By 
erecting  lightning-rods,  we  thus  lessen  the  liability  of  a  sudden  discharge. 
Every  drop  of  rain,  and  every  snow-flake,  falls  charged  with  electric  energy, 
and  thus  quietly  disarms  the  clouds  of  their  terror.  The  balls  of  electric 
light,  called  by  sailors  "  St.  Elmo's  fire,"  which  sometimes  cling  to  the  masts 
and  shrouds  of  vessels  and  the  flames  said  to  play  about  the  points  of 
bayonets,  indicate  the  quiet  escape  of  electricity  from  the  earth  toward  the 
clouds. 


312  ELECTRICITY. 

lightning,  seem  motionless  ;    while  a  cannon-ball,  in 
swift  flight,  appears  poised  in  mid-air. 

17.  Effects  of  Frictional  Electricity.—  (1.)  PHYS- 
ICAL. —  Discharges  from  a  large  battery  of  Leyden 
jars  will  melt  metal  rods,  perforate  glass,  split  wood, 
magnetize  steel  bars,  etc.  —  Let  a  person  stand  upon 
an  insulated  stool  and  become  charged  from  the 
prime  conductor.  His  hair,  through  repulsion,  will 
stand  erect  in  a  ludicrous  manner.  On  presenting 
his  hand  to  a  little  ether  contained  in  a  warm  spoon, 
a  spark  leaping  from  his  extended  finger  will  ignite 


FIG.  213. 


Rod  of  Steel  ready  for  Magnetization. 


it.  If  he  hold  in  his  hand  an  icicle,  the  spark  will 
readily  dart  from  it  to  the  liquid.*  —  A  card  held  be- 
tween the  knob  of  a  Leyden  jar  and  that  of  the  dis- 
charger, will  be  punctured  by  the  spark.  —  A  piece  of 
steel  may  be  magnetized  by  the  discharge  from  a 
Leyden  jar.  Wind  a  covered  copper  wire  around  a 
steel  bar,  as  in  Fig.  213,  or  inclose  a  needle  in  a 
small  glass  tube,  around  which  the  wire  may  be 
wound.  On  passing  the  spark  through  the  wire,  the 
needle  will  attract  iron  filings.  —  When  strips  of  tin- 
foil are  pasted  on  glass,  and  figures  of  various  pat- 
terns cut  from  them,  the  electric  spark  leaping  from 

*  This  experiment  can  be  more  surely  performed  by  using  disulphide 
of  carbon.  The  insulating  stool  may  be  merely  a  board  laid  on  four  dry 
flint-glass  bottles  or  goblets,  and  the  electricity  be  developed  by  rubbing  a 
glass  tube. 


VOLTAIC     ELECTKICITY. 


313 


FIG.  214. 


one  to  the  other  presents  a  beautiful  appearance. — If 
a  battery  be  discharged  through  a  small  wire  the 
electricity  will  be  changed  to  heat, 
and  the  wire,  if  sufficiently  small, 
will  be  fused  into  globules  or  dissi- 
pated in  smoke. 

(2.)  CHEMICAL  EFFECTS. — The 
"electric  gun"  is  filled  with  a  mixt- 
ure of  oxygen  and  hydrogen  gases. 
A  spark  causes  them  to  combine 
with  a  loud  explosion  and  form 
water. — The  sulphurous  smell  which 
accompanies  the  working  of  an  electrical  machine, 
and  is  noticed  in  places  struck  by  lightning,  is 
owing  to  the  production  of  ozone,  a  peculiar  form  of 
the  oxygen  of  the  air.  (See  "Popular  Chemistry," 
p.  23.) 

(3.)  PHYSIOLOGICAL  EFFECTS. — A  slight  charge  from 
a  Leyden  jar  produces  a  contraction  of  the  muscles 
and  a  spasmodic  sensation  in  the  wrist.  A  stronger 
one  becomes  painful  and  even  dangerous. 


Illuminated  Pane. 


II.    VOLTAIC   ELECTRICITY.* 

1.  Simple  Voltaic  Circuit.  — If  a  strip  of  zinc, 
coated  over  with  mercury,  be  put  into  a  mixture  of 
sulphuric  acid  and  water,  no  perceptible  chemical 
action  will  be  noticed.  But  if  a  strip  of  copper  or 


*  This  name  is  given  in  honor  of  the  Italian  physicist  who  made  the 
first  discoveries  in  this  branch  of  electricity. 


314  ELECTRICITY. 

platinum  be  immersed  at  the  same  time,  and  the 
upper  ends  of  the  two  pieces  of  metal  be  touched 
together  or  connected  by  wires,  many  little  bubbles 
of  gas  will  be  seen  on  the  second  strip,  forming  and 
rising  to  the  surface.  When  the  experiment  is  per- 
formed in  the  dark,  an  almost  in- 

FIG   215. 

finitesimal  spark  is  perceived  at 
the  moment  the  wires  are  joined.* 
Two  metal  plates  joined  in  this 
way  form  a  voltaic  pair.  The  ex- 
posed end  of  the  copper  or  platinum 
plate  is  called  the  positive  pole,  and 
that  of  the  zinc  the  negative  pole 
of  the  pair,  f  Joining  the  wires. 

A  Voltaic  Pair. 

or  otherwise  connecting  the  poles, 
is  termed  closing  the  circuit;  and  separating  them, 
breaking  the  circuit.  A  cup  prepared  for  such  an  ex- 
periment is  called  a  voltaic  cell. 

2.  Action  in  the  Voltaic  Cell. — Zinc  is  far  more 
easily  acted  upon  by  sulphuric  acid  than  copper  is. 
Each  molecule  of  the  acid  is  composed  of  two  atoms 
of  hydrogen,  one  of  sulphur,  and  four  of  oxygen.  J 
This  maybe  expressed  by  the  symbol,  H2S04.  When 
the  acid,  mixed  with  water,  acts  on  zinc  (Zn),  its  hy- 
drogen is  set  free,  and  a  new  substance,  called  zinc 

*  We  can  easily  form  a  simple  galvanic  circuit  by  placing  a  silver  coin 
between  our  teeth  and  upper  lip,  and  a  piece  of  zinc  under  our  tongue. 
On  pressing  the  edges  of  the  two  metals  together,  a  peculiar  taste  will  be 
perceived. 

t  These  names  may  easily  be  remembered  if  we  associate  the  p's  with 
copper  and  positive,  and  the  n's  with  zinc  and  negative. 

t  For  the  properties  of  these  elements,  refer  to  "Popular  Chemistry," 
pp.  11,  38,  and  103. 


VOLTAIC    ELECTKICITY.  315 

sulphate,  is  produced.  Its  symbol  is  Zn  S04.  It  is  at 
once  dissolved  in  the  water,  leaving  a  fresh  surface 
of  metal  to  be  attacked  by  the  acid.  It  is  thought 
that  each  molecule  of  liquid  between  the  copper  and 
zinc  becomes  polarized,  then  decomposed,  giving  up 
its  H2  to  its  neighbor  on  the  side  toward  the  copper, 
and  its  S04  to  its  neighbor  on  the  side  toward  the 
zinc.  The  H3  liberated,  in  contact  with  the  copper, 
gathers  in  bubbles  of  gas  ;  the  S04,  in  contact  with 
the  zinc,  unites  with  this  metal  to  form  zinc-sulphate, 
which  dissolves  in  the  liquid  of  the  cell.  If  the  ex- 
posed ends  of  the  two  plates  be  examined  with  a  suf- 
ficiently delicate  electroscope,  while  they  are  still 
separate,  it  is  found  that  the  copper  is  electrically  at 
higher  potential  than  the  zinc.  When  they  are  con- 
nected, neutralization  instantly  takes  place,  but  the 
action  of  the  acid  on  the  zinc  renews  the  difference  of 
potential  between  the  copper  and  the  zinc,  so  that  the 
process  continues  as  long  as  this  action  continues.* 

3.  The  Electric  Current.— The  term  "current"  is 
applied  to  this  continuous  neutralization  and  renewal 
of  difference  of  potential  in  the  closed  voltaic  circuit. 
The  current  is  said  to  "  flow  "  through  the  conduct- 
ing wire  from  the  copper  at  high  potential  toward  the 
zinc  at  low  potential,  just  as  water  flows  from  an 

*  With,  what  inconceivable  rapidity  must  these  successive  changes  take 
place  in  an  iron  wire  to  transmit  the  electric  energy,  as  in  actual  experi- 
ments, from  Valentia,  Ireland,  across  the  bed  of  the  Atlantic  and  the 
American  continent  to  San  Francisco  and  return,  a  distance  of  14,000 
miles,  in  two  minutes  !  In  fact,  it  far  surpassed  the  velocity  of  the  earth's 
rotation,  by  which  we  measure  time  and  leaving  Valentia  at  7:21  A.  M., 
Feb.  1,  it  reached  San  Francisco  at  11:20  p.  M.,  Jan.  31. 


816  ELECTRICITY. 

elevated  reservoir  through  a  pipe  toward  a  lower  reser- 
voir. There  is  no  actual  transfer  of  matter,  no  cur- 
rent of  fluid ;  but  only  by  analogy  we  may  call  it  a 
current  of  energy  transmitted  through  the  entire 
thickness  and  length  of  the  conducting  wire.  By 
analogy  also  the  current  is  said  to  pass  through  the 
cell  from  zinc  to  copper,  thus  completing  its  circuit. 

4.  The  Volt. — We  measure  the  difference  of  tem- 
perature between  two  bodies  in  degrees  on  the  ther- 
mometer scale,  or  the  difference  of  level  between  the 
surfaces  of  two  reservoirs  in  feet  or  meters.  These 
are  the  accepted  units  of  measurement.  In  like 
manner,  for  measuring  the  difference  of  potential 
between  two  bodies,  a  unit  called  the  volt*  has  been 
selected.  In  the  simple  voltaic  cell  already  described, 
when  freshly  set  in  action,  the  difference  of  poten- 
tial between  its  poles  is  about  one  volt.  The  force 
due  to  difference  of  potential  is  called  electro-motive 
force,  and  is  always  measured  in  volts,  f 

*  Named  in  honor  of  Alessandro  Volta,  an  Italian  physicist,  who  was 
born  in  1745.  In  1793,  he  communicated  to  the  Royal  Society  of  London 
an  account  of  his  important  experiments  on  which  the  modern  science  of 
electricity  has  been  largely  built. 

.  t  The  difference  of  potential  between  the  discharging  rods  of  a  Voss 
electrical  machine  when  giving  long  sparks  is  often  several  hundreds  of 
volts.  When  passed  through  the  body,  such  momentary  currents  are  pain- 
ful. The  potential  of  the  air  during  a  thunder-storm  quickly  changes 
through  thousands  of  volts.  The  voltaic  cell  furnishes  a  current  that  is 
exceedingly  steady  in  comparison  with  the  stream  of  sparks  from  an  elec- 
trical machine,  but  of  only  small  electro-motive  force.  Frictional  electricity 
is  sudden,  noisy,  convulsive;  voltaic  is  gentle,  silent,  yet  powerful.  The 
one  is  like  a  quick,  violent  blow,  as  of  a  swiftly  moving  bullet ;  the  other  like 
the  steady  uniform  pressure  produced  by  a  large  mass  slowly  advancing 
against  resistance.  Lightning  leaps  across  miles  of  air ;  the  voltaic  current 
will  pass  through  a  conductor  from  England  to  California  rather  than 
spark  across  half  an  inch  of  air.  The  most  powerful  frictional  machine 


VOLTAIC    ELECTRICITY.  317 

5.  Electrical    Resistance. —  Every   conductor  op- 
poses resistance  to  the  electric  current.    The  amount 
of  resistance  offered  depends  upon  the  nature  of  the 
conductor,  its  length  and  cross-section.    Metals  offer 
much  less  resistance  than  liquids,  and  these  again  less 
than  substances  classed  as  insulators  (§7,  p.  298).    For 
the  same  material  the  resistance  increases  with  its 
length  and  decreases  with  increase  of  its  cross-section. 

6.  The  Ohm. — To  measure  resistance,  a  unit  called 
the  ohm*  has  been  selected.     A   piece  of  common 
copper  wire,  as  thick  as  the  band  shown  in  Fig.  216, 
and  fifty  yards  long,  opposes  a  resist-  FIG.  210. 
ance  of  about  one  ohm.    Coils  whose 

resistance  in  ohms  is  known  are  much  used  in  elec- 
trical measurement. 

7.  The   Ampere. — To  measure   the   effective   cur- 
rent  strength   obtained   from   a   voltaic   cell,   a  unit 
called    an    ampere  \    has    been    selected.      It    is    the 

would  be  insufficient  for  telegraphing ;  while  signals  have  been  sent  across 
the  ocean  with  a  tiny  battery  composed  of  "  a  gun-cap  and  a  strip  of  zinc, 
excited  by  a  drop  of  water  the  bulk  of  a  tear."  "  Faraday  immersed  a 
voltaic  pair,  composed  of  a  wire  of  platinum  and  one  of  zinc,  in  a  solution 
of  four  ounces  of  water  and  one  drop  of  oil  of  vitriol.  In  three  seconds  this 
produced  as  great  a  deviation  of  the  galvonometer  needle  as  was  obtained 
by  30  turns  of  the  powerful  plate-glass  machine.  If  this  had  been  concen- 
trated in  one  millionth  of  a  second,  the  duration  of  an  electric  spark,  it 
would  have  been  sufficient  to  kill  a  cat ;  yet  it  would  require  800,000  such 
discharges  to  decompose  a  grain  of  water." 

*  Named  in  honor  of  Dr.  Q-.  S.  Ohm,  a  German  physicist,  who  deter- 
mined the  relation  existing  between  current  strength,  electro-motive  force, 
and  resistance. 

t  Named  for  Andre  Marie  Ampere,  a  French  physicist,  born  in  1775, 
whose  splendid  work  in  electricity  was  such  as  to  give  him  the  highest 
rank  along  with  Volta. 

The  definitions  of  electrical  units  given  in  the  text  are  not  exact 
enough  to  furnish  more  than  the  most  elementary  ideas.  The  student  will 


318  ELECTRICITY. 

amount  of  current  obtained  when  one  volt  of  electro- 
motive force  acts  against  one  ohm  of  resistance. 

With  these  units  electricity  can  be  measured  with 
as  much  exactness  as  we  measure  quantities  of  grain 
or  water. 

8.  A  Battery  consists  of  two  or  more  voltaic  cells 
so  connected  as  to   secure  a  stronger  current  than 
can  be  obtained  from   a  single   cell.     According  to 
Ohm's  Law,  the  current  strength  (C)  in  amperes  is 
equal  to  the  electro-motive  force  (E)  in  volts  divided 
by  the  resistance  in  ohms.    The  resistance  is  partly 
in   the    external    conductor    (R)    and    partly    in    the 
liquid  of  the  battery  (r).     The  law  is  expressed  in  a 
formula,  thus, 

c_      E 

~  R  +  r 

With  a  large  number  of  cells  a  battery,  therefore, 
can  be  arranged  either  to  overcome  a  large  external 
resistance,  or,  when  this  is  small,  to  furnish  a  strong 
current. 

9.  Polarization  within  the   Battery.— As  soon  as 
the   action  of  a  battery  is  well  begun,  the   electro- 
motive   force    becomes    rapidly    diminished   because 
hydrogen  tends  to  collect  upon  the  plate  in  connec- 
tion with  the   positive  pole.     The  bubbles  interfere 
with    further    action    and    start    a    counter   electro- 
motive   force    which    neutralizes   much    of   that   in 
operation.     Many   different   devices   have    been    em- 
find  them  more  accurately  defined  in  any  text-book  devoted  specially  to 
Electricity,  such  as  that  of  Thompson  or  Urbanitzky. 


VOLTAIC     ELECTRICITY. 


319 


FIG.  217. 


ployed  to  diminish  this  evil,  and  each  gives  rise  to  a 
special  kind  of  battery.  Only  a  few  need  be  de- 
scribed. 

1O.  The  Potassium  Bichromate  Battery.— Instead 
of  copper,  a  pair  of  plates  of  car^ 
bon  are  immersed,  with  a  plate  of 
zinc  between  them,  arranged  so 
as  to  slide  into  the  liquid  or  out 
of  it  at  will.  A  solution  of  po- 
tassium bichromate  in  sulphuric 
acid  and  water  is  used.  The  sul- 
phuric acid  acts  on  the  zinc,  and 
the  hydrogen  is  prevented  from 
forming  in  bubbles  by  being  com- 
bined at  once  with  some  of  the 
oxygen  which  the  chromic  acid 
yields. 

DANIELL'S    BATTERY.  —  In    this 

battery  there  are  two  fluids  separated  by  a  cup  of 
porous  earthenware,  which  does  not  prevent  the  pas- 
sage of  'the  current.  In  the  outer 
vessel  of  glass  there  is  a  strong  solu- 
tion of  copper  sulphate  (CuS04)  in 
which  a  split  copper  cylinder  is  im- 
mersed. Within  this  is  placed  the 
porous  cup,  containing  a  rod  of  zinc 
coated  with  mercury  and  a  mixture 
of  sulphuric  acid  with  water.  Zinc 
sulphate  is  produced,  and  the  liber- 
ated hydrogen  decomposes  some  of  the  copper  sul- 
phate, taking  its  S04  and  causing  a  deposit  of  metal- 


Potassium  Bichromate  Cell. 


FIG.  218. 


Daniell  Cell. 


320 


E  L  E  C  T  R  L  C  1 T  Y  . 


PIG.  219. 


lie  copper.    Polarization  is  thus  prevented,  and  this 
is  one  of  the  most  constant  batteries  known. 

GROVE'S  BATTEEY. — In  this  the  outer  cup  contains 
the  zinc  and  dilute  sulphuric  acid.    With- 
in the  porous  cup  a  strip  of  platinum  dips 
into  strong  nitric  acid.    The  hydrogen  de- 
composes some  of  the  nitric  acid,  taking 
oxygen  from  it  to  produce  water  and  lib- 
erating red  fumes  of  nitrogen  tetroxide, 
which  are  unpleasant  and  hurtful.    This 
battery  gives  very  high  electro-motive  force. 
BUNSEN'S   BATTERY. — In   this   rods   of   carbon   are 
substituted  for  the  strips  of  platinum  used  in   the 
Grove    battery.      Sometimes    potassium    bichromate 

Fro.  220 


Grove  Cell. 


Bunsen  Battery. 

solution  is  substituted  for  nitric  acid  in  order  to 
avoid  the  production  of  nitrous  fumes.  Fig.  220 
shows  a  Bunsen  battery  arranged  in  series. 

11.  Effects  of  Voltaic  Electricity.— (1.)  PHYSICAL. 
— If  a  current  of  electricity  is  passed  through  a  wire 
too  small  to  conduct  it  readily,  it  is  converted  into 


VOLTAIC     ELECTRICITY. 


321 


FIG.  221. 


heat.  The  poorer  the  conducting  power  of  the  wire, 
and  hence  the  greater  the  resistance,  the  more 
marked  the  effect.  With  ten  or  twelve  Grove's  cups 
several  inches  of  fine  steel  wire  may  be  fused ;  and 
with  a  powerful  battery,  several  yards  of  platinum 
wire  may  be  made  to  glow  with  very  brilliant  effect, 
giving  a  steady  light.* 

In  closing  or  breaking  the  circuit,  we  produce  a 
spark,  the  size  of  which  depends  on  the  electro- 
motive force  and  cur- 
rent strength  of  the 
battery.  With  several 
cells,  beautiful  scintil- 
lating sparks  are  ob- 
tained by  fastening 
one  pole  to  a  file  and 
rubbing  the  other  up- 
on it.  When  charcoal 
or  gas -carbon  elec- 
trodes are  used  with 
a  powerful  battery,  on 
slightly  separating  the 
points,  the  intervening 
space  is  spanned  by  an  arch  of  the  most  dazzling 
light  (Fig.  221).  The  flame,  reaching  out  from  the 
positive  pole  like  a  tongue,  vibrates  around  the  neg- 
ative pole,  licking  now  ^  on  this  side  and  now  on  that. 
The  heat  is  intense.  Platinum  melts  in  it  like  wax 

*  Torpedoes  and  blasts  are  fired  on  this  principle.  Two  copper  wires 
leading  from  the  battery  to  the  spot  are  separated  in  the  powder  by  a  short 
piece  of  small  steel  wire.  When  the  circuit  is  completed,  the  fine  wire  be- 
comes red-hot  and  explodes  the  charge, 


The  Arc  Light. 


322  ELECTRICITY. 

in  the  flame  of  a  candle,*  the  metals  burn  with  their 
characteristic  colors  ;  and  lime,  quartz,  etc.,  are  fused. 
The  effect  is  not  produced  by  burning  the  charcoal 
points,  since  in  a  vacuum  it  is  equally  brilliant. 

(2.)  CHEMICAL  EFFECTS. — Electrolysis  (to  loosen  by 
electricity)  is  the  process  of  the  decomposition  of 
compound  bodies  by  the  voltaic  current.  If  plati- 
num electrodes  be  held  a  little  distance  apart  in  a 
cup  of  water  mixed  with  sulphuric  acid,  tiny  bubbles 
will  immediately  begin  to  rise  to  the  surface.  When 
the  gases  are  collected,  they  are  found  to  be  oxygen 
and  hydrogen,  in  the  proportion  of  two  volumes  of 
the  latter  to  one  of  the  former,  f  In  the  electrolysis  of 

*  To  show  the  varying  conducting  power  of  the  different  metals,  fasten 
together  alternate  lengths  of  silver  and  platinum  wire  and  pass  the  current 
through  them.  The  latter  will  glow,  while  the  former,  conveying  the  elec- 
tricity more  perfectly,  will  scarcely  manifest  its  presence. 

There  are  two  forms  of  the  electric  light  now  used— the  arc  (shown  in 
Pig.  221),  where  the  current  passes  between  two  carbon  points;  and  the 
incandescent,  where  the  current  heats  to  a  dazzling  white  a  carbon  strip 
placed  in  the  circuit.  The  former  is  employed  in  lighting  streets,  railroad 
stations,  and  large  halls ;  the  latter  is  generally  used  in  dwellings,  etc.,  as  it 
gives  a  softer  light,  and  is  much  more  steady.  Edison's  Lamp  consists  of  a 
tiny  carbon  loop  placed  in  a  glass  globe  from  which  the  air  has  been  so 
completely  exhausted  as  to  leave  only  i  tioo  ooo  of  an  atmosphere.  When 
exposed  to  the  air,  the  voltaic  arc  rapidly  wastes  the  carbon  points.  Elec- 
tric lamps  have  therefore  been  devised  that,  by  a  self-acting  apparatus, 
keep  the  points  at  a  proper  distance  from  each  other. 

t  If  the  copper  poles  be  inserted,  bubbles  will  pass  off  from  the  nega- 
tive, but  none  from  the  positive  pole,  since  the  oxygen  combines  with  the 
copper  wire.  That  gas  has  no  effect  on  platinum.  The  burning  of  an  atom 
of  zinc  in  the  battery  develops  enough  electricity  to  set  free  an  atom  of 
oxygen  at  the  positive  pole.  It  is  interesting  to  notice  that  in  the  battery 
there  is  zinc  burning,  i.  e.,  combining  with  oxygen,  but  without  light  or 
heat ;  in  the  electric  light  the  real  force  of  the  combustion  is  revealed.  We 
may  thus  transfer  the  light  and  heat  to  a  great  distance  from  the  place 
where  they  take  their  origin.  The  transmission  of  energy  thus  to  a  distance 
is  better  effected  through  electricity  than  through  any  other  agency.  Much 
ingenuity  has  been  expended  on  machines  for  this  purpose. 


VOLTAIC    ELECTRICITY. 


323 


compounds,  their  elements  are  found  to  be  in  differ- 
ent electrical  conditions.  Hydrogen  and  most  of  the 
metals  go  to  the  negative  pole,  and  are  electro-posi- 
tive. Oxygen,  chlorine,  sulphur,  etc.,  go  to  the  posi- 
tive pole,  and  are  therefore  electro-negative. 

On  disconnecting  the  electrodes  of  the  voltameter 
(Fig.  2^22)  from  the  battery  and  joining  them  with  a 
conductor,  a  current  passes  through  the  conductor 


FIG.  222. 


H   0 


from  the  electrode  covered  with  oxygen  to  that  cov- 
ered with  hydrogen.  A  voltameter  thus  charged  con- 
stitutes a  secondary  cell  in  which  the  energy  of  a 
current  may  be  stored  up  and  again  given  out.* 

Electrotyping  is  the  process  of  depositing  metals 
from  their  solutions  by  electricity.  It  is  use'd  in 

*  Faure's  accumulator  consists  of  two  lead  plates  coated  with  red  lead, 
rolled  together  with  flannel  between  them,  and  immersed  in  dilute  sulphuric 
acid.  A  current  of  electricity  passed  through  such  a  cell  changes  a  part  of 
the  red  lead  on  the  positive  plate  into  peroxide  of  lead ;  and  a  part  of  the  red 
lead  on  the  negative  plate  into  spongy  metallic  lead.  A  battery  of  these  cells 
when  freshly  charged  will  retain  its  energy  and  produce  a  sustained  current 
when  desired. 


324 


ELECTRICITY. 


copying  medals,  wood-cuts,  types,  etc.  An  impres- 
sion of  the  object  is  taken  with  gutta-percha  or 
wax.  The  surface  to  be  copied  is  brushed  with 
black-lead  to  render  it  a  conductor.  The  mold  is 
then  suspended  in  a  solution  of  copper  sulphate, 
from  the  negative  pole  of  the  battery,  and  a  plate 

PIG.  223 


Electrotyping. 

of  copper  is  hung  opposite  on  the  positive  pole. 
The  electric  current  decomposes  the  copper  sul- 
phate ;  the  metal  goes  to  the  negative  pole  and  is 
deposited  upon  the  mold,  while  the  acid,  passing  to 
the  positive  pole,  dissolves  the  copper,  and  preserves 
the  strength  of  the  solution.* 

*  While  the  plate  is  hanging  in  the  solution  there  is  no  noise  heard  or 
bubbling  seen.  The  most  delicate  sense  fails  to  detect  any  movement.  Yet 
the  mysterious  electric  force  is  continually  drawing  particles  of  ruddy,  solid 
copper  out  of  the  blue  liquid,  and,  noiselessly  as  the  fall  of  snow-flakes,  drop- 
ping them  on  the  mold ;  producing  a  metal  purer  than  any  chemist  can 
manufacture,  spreading  it  with  a  uniformity  no  artist  can  attain,  and 
copying  every  line  with  a  fidelity  that  knows  no  mistake. 


VOLTAIC     ELECTRICITY.  325 

Electro-elating  is  the  process  of  coating  with  sil- 
ver or  gold  by  electricity.  The  metal  is  readily  de- 
posited on  German  silver,  brass,  copper,  or  nickel 
silver  (a  mixture  of  copper,  zinc,  and  nickel).  The 
objects  to  be  plated  are  thoroughly  cleansed,  and 
then  hung  from  the  negative  pole  in  a  solution  of 
silver,  while  a  plate  of  silver  is  suspended  on  the 
positive  pole.  In  five  minutes  a  "  blush "  of  the 
metal  will  be  deposited,  which  conceals  the  other 
metal  and  is  susceptible  of  polish.* 

(3.)  PHYSIOLOGICAL  EFFECTS. — With  a  single  cell  no 
special  sensation  is  experienced  when  the  two  poles 
are  held  in  the  hands.  With  a  large  battery  a  sud- 


A  *  Place  in  a  large  test-tube  a  silver  coin  with  a  little  nitric  acid.  If 
the  fumes  of  the  decomposed  acid  do  not  soon  rise,  warm  the  liquid. 
When  the  silver  is  dissolved,  fill  the  tube  nearly  full  of  soft  water.  Next 
drop  hydrochloric  acid  into  the  liquid  until  the  white  precipitate  (silver 
chloride)  ceases  to  fall.  When  the  chloride  has  settled,  pour  off  the  colored 
water  which  floats  on  top.  Pill  the  tube  again  with  soft  water ;  shake  it 
thoroughly ;  let  it  settle,  and  then  pour  off  as  before.  Continue  this  proc- 
ess until  the  liquid  loses  all  color.  Finally,  fill  with  water  and  heat  mod- 
erately, adding  potassium  cyanide  (the  pupil  will  remember  that  this  sub- 
stance is  exceedingly  poisonous)  in  small  bits  as  it  dissolves,  until  the 
chloride  is  nearly  taken  up.  The  liquid  is  then  ready  for  electro-plating. 
Thoroughly  cleanse  a  brass  key,  hang  it  from  the  negative  pole  of  a  small 
battery,  and  suspend  a  silver  coin  from  the  positive  pole.  Place  these  in 
the  silver  solution,  very  near  and  facing  each  other.  When  well  whitened 
by  the  deposit  of  silver,  remove  the  key  and  polish  it  with  chalk.  In  the 
arts  the  polishing  is  performed  by  rubbing  with  "burnishers."  These  are 
made  of  polished  steel,  and  fit  the  surfaces  of  the  various  articles  upon 
which  they  are  to  be  used.  It  is  said  that  an  ounce  of  silver  can  be  spread 
over  two  acres  of  surface.  A  well-plated  spoon  receives  about  as  much  sil- 
ver as  there  is  In  a  ten-cent  piece.  The  only  method  of  deciding  accurately 
the  amount  deposited  is  by  weighing  the  article  before  and  after  it  is 
plated.— A  vessel  may  be  "gold-lined"  by  filling  it  with  a  solution  of  gold, 
suspending  in  it  a  slip  of  gold  from  the  positive  pole  of  the  battery,  and 
then  attaching  the  negative  pole  to  the  vessel.  The  current  passing 
through  the  liquid  causes  it  to  bubble  like  soda-water,  and  in  a  few  mo- 
ments deposits  a  thin  film  of  gold  over  the  entire  surface. 


326 


ELECTRICITY. 


den  twinge  is  felt,  and  the  shock  becomes  painful 
and  even  dangerous,  especially  if  the  palms  are 
moistened  with  salt  or  acid  water  to  increase  the 
conduction.  Rabbits  which  had  been  suffocated  for 
half  an  hour,  have  been  restored  by  an  application 
of  a  strong  voltaic  current. 


III.    TRANSFORMATIONS  OF  ELECTRIC  ENERGY. 

1.  Effect  of  a  Voltaic  Current  on  a  Magnetic 
Needle. — If  a  wire  conducting  an  electric  current  be 
placed  over  a  poised  magnetic  needle,  this  tends  to 
place  itself  at  right  angles  to  the  wire. 


FIG.  224. 


Effect  of  Current  on  Needle. 


Assuming  the  direction  of  the  current  to  be 
northward,  the  north  pole  of  the  needle  will  be 
turned  toward  the  left.  The  same  effect  will  be 
produced  if  the  current  pass  southward  under  the 


TRANSFORMATIONS     OF     ENERGY. 


327 


needle,  or  vertically  downward  on  the  north  side  of 
it,  or  vertically  upward  on  the  south  side  of  it. 
By  reversing  these  conditions,  the  north  pole  of  the 
needle  will  be  turned  toward  the  right.  The  play  of 
the  needle  becomes  thus  a  test  of  the  presence  and 
direction  of  an  electric  current.*  The  delicacy  of 

FlG.  225. 


\ 

Pivoted  Hoop  of  Conducting  Wire. 

this  test  is  greatly  increased  if  the  wire,  properly 
insulated,  be  coiled  into  a  ring  with  many  turns,  at 
the  center  of  which  the  needle  is  pivoted  or  sus- 
pended. 

2.  A  Wire  bearing  a  Current  acts  like  a  Mag- 
net.— Let  a  copper-wire  hoop  be  pivoted,  as  shown 
in  Fig.  225,  so  that  its  plane  is  in  a  north  and  south 

*  Ampere  gave  a  very  convenient  rule  for  determining  the  direction 
of  the  current  from  the  motion  of  the  magnetic  needle.  Imagine  the  cur- 
rent to  be  like  a  stream  of  water,  with  a  little  swimmer  in  it,  facing  the 
needle  and  swimming  along  with  the  current.  The  north  pole  of  the  needle 
will  always  turn  toward  his  left.  The  pupil  should  try  the  experiment  and 
test  it  in  all  possible  ways. 


328 


ELECTRICITY. 


FIG.  226. 


direction.  On  passing  an  electric  current  through  the 
hoop  it  slowly  turns  until  its  plane  assumes  an  east 
and  west  position  and  its  axis  points  north  and  south. 
Evidently  the  current-circle  behaves  like  a  magnet ; 

one  side  of  it  being  north, 
the  other  south-magnetic. 
A  more  effective  arrange- 
ment consists  of  a  wire 
wound  spirally  as  shown  in 
Fig.  226.  A  wire  so  wound 
is  called  a  solenoid.  On  pass- 
ing a  current  through  the 

solenoid  it  promptly  places  itself  with  its  axis  in  the 
magnetic  meridian.  With  a  pair  of  such  solenoids  (see 
Fig.  227)  the  experiments  relating  to  the  attraction  and 

FIG.  227. 


Pivoted  Helix  of  Wire. 


Two  Helices  acting  like  Magnets. 

repulsion  of  poles  of  magnets  may  be  repeated.  The 
polarity  of  the  solenoid  depends  upon  the  direction  in 
which  the  current  passes  through  it,  and  may  be  deter- 
mined by  the  following  rule  :  If  on  looking  along  the 
axis  of  the  solenoid  the  current  runs  through  it  in  a 


TRANSFORMATIONS    OF    ENERGY.  329 

direction  contrary  to  the  direction  of  the  hand  of  a 
watch  (viewed  face  up),  the  observer  is  looking  at  the 
north  end  of  the  solenoid. 

FIG.  228. 


The  Tangent  Galvanometer. 

3.  The  Tangent  Galvanometer.— Any  instrument 
designed  to  measure  the  length  of  an  electric  cur- 
rent is  called  a  galvanometer.  Of  the  many  varieties, 
the  tangent  galvanometer  is  the  most  important.  It 
consists  of  one  or  more  coils  of  insulated  wire  wound 
upon  a  wooden  hoop,  at  the  center  of  which  a  small 


330 


ELECTRICITY. 


PlO.  229. 


magnetic  needle  is  pivoted  or  suspended  (Fig.  228). 
The  plane  of  the  coils  is  made  to  coincide  with  that 
of  the  magnetic  meridian.  The  earth's  magnetism 
tends  to  keep  the  needle  in  this  plane. 
The  magnetic  effect  of  a  current 
tends  to  make  it  assume  a  position 
across  this  plane.  Obeying  both  forces 
it  assumes  an  oblique  position,  so  as 
to  make  a  measurable  angle  with  the 
meridian.  The  strength  of  current  is 
proportional  to  the  tangent  of  this 
angle. 

4.      The     Electro-magnet. — If    a 

current  be  passed  through  a  coil  held 
vertically  (Fig.  229),  a  rod  of  soft 
iron  placed  below  will  be  drawn  up 
into  the  coil,  springing  up  as  if  en- 
dowed with  life  at  the  moment  the 
A  Magnetic  Mahomet's  current  begins.  It  drops  as  soon  as 

Coffin- 

the  circuit  is  broken.*  Let  a  pair  of 
such  coils  be  fixed  around  the  arms  of  a  U-shaped 
rod  of  soft  iron.  This  becomes  a  strong  horseshoe 
magnet,  whose  strength  comes  and  goes  as  the  cur- 
rent is  made  or  broken.  It  is  therefore  called  an 
electro-magnet.  Such  magnets  have  been  made 
strong  enough  to  sustain  a  weight  of  several  tons 
attached  to  the  armature  below. 

5.   The  Electro-magnetic  Telegraph  depends  on 


*  Thus  is  realized  in  science  the  fabulous  story  of  Mahomet's  coffin, 
which  is  said  to  have  been  suspended  in  mid-air. 


TRANSFORMATIONS    OF    ENERGY. 


331 


FIG.  230. 


the  principle  of  closing  and  breaking  the  circuit  at 
one  station,  and  thereby  making  and  unmaking  an 
electro-magnet  at  the  station 
with  which  communication  is 
held.  A  single  wire  connects 
the  two  stations  and  is  joined 
at  each  station  to  a  key,  a  reg- 
ister (sounder),  and  a  battery. 
One  pole  of  each  battery  is  con- 
nected  with  the  ground.  When 
a  current  is  sent  along  the  wire 
the  circuit  is  completed  through 
the  earth.  The  key  is  used  for 
sending  messages  ;  the  register  for  receiving  them. 

FIG.  231. 


The  Electro-magnet. 


The  Telegraph  Key. 

The  key  is  shown  in  Fig.  231.  E  and  F  are 
screws  which  fasten  the  instrument  to  the  table, 
and  also  hold  the  two  ends  of  the  wire.  F  is  insu- 


332  ELECTRICITY. 

lated  by  a  ring  of  vulcanite  where  it  passes  through 
the  table  and  the  metal  plate  B.  H  is  a  lever  with 
a  finger-button  G,  a  spring  /,  to  keep  it  lifted,  and  a 
screw  Dj  to  regulate  the  distance  it  can  move.  At 
A  is  a  break  between  two  platinum  points,  which 
form  the  real  ends  of  the  wires.  When  Gr  is  de- 
pressed, the  circuit  is  complete,  and  when  lifted,  it 
is  broken.  C  is  a  circuit-closer  that  is  used  when 
the  key  is  not  in  operation ;  the  arm  being  pushed 
under  A  touches  the  platinum  wire,  and  so  com- 
pletes the  circuit.  It  is  pushed  out  whenever  the 
operator  manipulates  @.  Then,  by  moving  G-,  he 
can  "close"  or  "open"  the  circuit  at  pleasure.  He 
thus  sends  a  message. 


The  Register. 


The  register  contains  an  electro-magnet,  E  (Fig. 
232).  When  the  circuit  is  complete,  the  current, 
passing  through  the  coils  of  wire  at  E,  attracts  the 
armature  m.  This  elevates  n,  the  other  end  of  the 
lever  mn,  "and  forces  the  rounded  point  x  firmly 
against  the  soft  paper  a.  As  soon  as  the  circuit  is 
broken,  E  ceases  to  be  a  magnet,  and  the  spring  R 
lifts  the  armature,  drawing  the  point  from  the  paper. 


TRANSFORMATIONS     OF     ENERGY. 


833 


Clock-work  attached  to  the  rollers  at  z  moves  the 
paper  along  uniformly  beneath  the  point  x.  When 
the  circuit  is  completed  and  broken  again  instantly, 
there  is  a  short  dot  made  on  the  paper.  This  is 
called  e  ;  two  dots,  i ;  three  dots,  s ;  four  dots,  h.  If 
the  current  is  closed  for  a  longer  time,  the  mark  be- 
comes a  dash,  t ;  two  dashes,  m ;  a  dot  and  a  dash,  a. 

TABLE    OF    MOKSE'S    SIGNS. 


a  •  — 

j    

s 

b  

k   

t    — 

c  .  .     . 

1 

u  •  «  — 

d  

m  

v   ...  — 

e  • 

n  —  • 

w  •  

f  

o    •  '  .  - 

x    .  —  " 

y. 

h  .... 

q    

z    •  -  -     « 

i    -  • 

r    •     •  •' 

&  .   ... 

A  skillful  operator  becomes  so  accustomed  to  the 
sound  that  the  clicking  of  the  armature  is  perfectly 
intelligible.  He  uses,  therefore,  simply  a  "sounder" 
i.e.,  a  register  without  the  paper  and  clock-work  at- 
tachment. Indeed,  the  register  has  now  gone  almost 
entirely  out  of  use,  and  every  operator  is  required 
to  read  by  sound. 

RELAY. — When  the  stations  are  more  than  fifty  or 
sixty  miles  apart,  the  current  becomes  generally  too 
weak  to  work  the  register.  By  substituting  the  relay 
for  it  in  the  line  circuit,  the  force  of  a  local  battery 
may  be  employed  to  work  the  sounder  or  register. 


334 


ELECTKICITY. 


In  Fig.  233,  which  represents  a  relay,  D  is  connected 
with  the  line  wire,  and  C  with  the  ground  wire ;  A 
is  connected  with  the  positive  pole  of  the  local  bat- 
tery, and  B  with  the  register  or  sounder,  and  thence 
with  the  negative  pole  of  this  battery.  The  main 
current  passes  in  at  D,  traverses  the  fine  wire  of 
the  electro-magnet,  K,  and  thence  passes  out  at  C  to 
the  ground.  The  armature  E,  playing  to  and  fro  as 

FIG.  233. 


The  Relay. 

the  current  from  the  distant  station  passes  through 
or  is  cut  off,  moves  the  lever  F.  This  works  on  an 
axis  at  the  lower  end  and  is  drawn  back  by  the 
spring  H,  which  is  regulated  by  the  thumb-screw  I. 
As  E  is  attracted,  the  circuit  at  G  is  closed ;  the 
current  from  A  traverses  a  wire  underneath,  up  F, 
and  down  L,  and  back  through  another  wire  under- 
neath to  B-,  thence  to  the  electro  -  magnet  of  the 
sounder,  which  therefore  attracts  its  armature. 

The  operator  who  sends  the  message  simply  com- 
pletes and  breaks  the  circuit  with  the  key ;  the  ar- 
mature of  the  relay,  at  the  station  where  the  mes- 


TRANSFORMATIONS     OF     ENERGY.  335 

sage  is  received,  vibrates  in  unison  with  these  move- 
ments ;  the  register  or  sounder  repeats  them  with 
greater  force ;  and,  the  second  operator  interprets 
their  meaning.* 

6.  Ocean  Cables. — The  Atlantic  and  Indian  Oceans 
have  been  spanned  with  cables  of  insulated  wire  for 
the  transmission  of  telegraphic  messages.  The  cable 
must  be  well  insulated  and  p^  334. 

very  strong.  In  the  middle  is 
a  bundle  of  copper  wires,  0 
(Fig.  234) ;  this  is  buried  with- 
in a  sheathing  of  gutta-percha, 
(7;  around  this  is  a  group  of 
cords  of  tarred  hemp,  H,  to 

protect  the  gutta-percha ;  and,  to  give  still  further 
protection  and  strength,  fifteen  or  twenty  iron  wires 
are  twisted  around  the  whole,  so  as  to  make  it  a 
rope  about  an  inch  thick. 

In  signaling  over  a  long  cable,  allowance  has  to 
be  made  for  the  great  resistance  of  the  long  wire, 
and  the  slowness  with  which  the  circuit  has  to  be 
operated.  Instead  of  using  a  relay  or  sounder,  it  is 
necessary  to  use  a  delicate  galvanometer  as  a  re- 
ceiver. A  beam  of  light  is  reflected  from  a  little 

*  The  simple  telegraph,  instrument  is  but  one  of  a  multitude  of  appli- 
cations that  have  been  made  of  the  electro-magnet.  By  various  ingenious 
devices  it  has  become  possible  to  send  two,  or  even  four,  messages  with 
reasonable  rapidity  over  the  same  line  at  the  same  time.  By  one  system, 
devised  by  Mr.  Delany,  as  many  as  seventy-two  circuits  have  been  operated 
with  a  single  instrument  at  the  rate  of  two  or  three  words  per  minute. 
The  message  is  often  printed  at  the  moment  it  is  received.  Prom  the  Stock 
Exchange  in  New  York  hundreds  of  printed  reports  are  thus  sent  at  the 
same  time  to  offices  in  various  parts  of  the  city. 


SB6 


ELECTRICITY. 


mirror  attached  to  the  magnetic  needle,  and  the 
swinging  of  the  bright  spot  on  a  screen  is  interpreted 
as  an  alphabet.  Or,  a  fine  glass  siphon  tube  is  at- 
tached to  a  movable  galvanometer  coil  which  swings 
between  the  poles  of  an  electro-magnet.  Its  short 
arm  dips  into  a  vessel  of  ink  which  is  insulated  and 
can  be  electrified.  The  long  arm  has  its  end  over  a 
strip  of  paper  moved  by  clock-work.  The  electrifica- 

FIG.  235. 


K 
Siphon-recorder  Alphabet. 

tion  of  the  ink  causes  it  to  issue  in  fine  drops  over 
the  moving  paper,  and  a  sinuous  line  is  recorded. 
This  "  Siphon-recorder "  alphabet  is  partly  shown  in 
Fig.  235. 

7.  Electro-Magnetic  Induction. —  An  electric  cur- 
rent or  a  magnet  produces  in  the  space  about  it  what 
is  called  a  field  of  force.  The  greater  the  strength  of 
the  current  or  magnet,  the  greater  is  the  force  in  the 
field.  The  farther  we  proceed  from  the  current  or 
magnet,  the  less  becomes  the  force.  A  closed  circuit  in 


TRANSFORMATIONS    OF    ENERGY. 


337 


which  no  battery  is  included,  placed  in  such  a  field  will 
have  a  current  induced  in  it  whenever  the  force  in  the 
field  acting  on  the  circuit  varies  in  strength.  When- 
ever, therefore,  the  inducing  current  is  made  or  broken, 
or  the  inducing  magnet  made  or  unmade,  or  when 
their  strength  is  simply  varied.  Relative  motion  of  the 
inducing  current  or  magnet  and  the  closed  circuit  will 

PIG.  236. 


Current  Induction. 

also  vary  the  force  acting  on  the  closed  circuit  and  pro- 
duce in  it  an  induced  current.  At  every  such  change  a 
momentary  current  is  induced  in  the  closed  circuit. 
Let  the  coil  P  (Fig.  236),  which  forms  a  closed  circuit 
with  the  battery,  be  thrust  into  the  coil  7,  which  forms 
a  closed  circuit  with  the  galvanometer.  Instantly  the 
needle  turns,  and  then  comes  back  to  rest.  Suddenly 
withdraw  P.  The  needle  turns  in  the  opposite  direc- 
tion, and  again  comes  back  to  rest.  P  is  called  the 
primary  coil ;  7,  the  secondary  or  induction  coil,  because 


338  ELECTRICITY. 

currents  in  it  are  induced  by  the  motion  of  P.  The 
current  in  /is  opposite  in  direction  to  that  of  Pwhen 
this  is  thrust  in,  and  in  the  same  direction  when  P  is 
withdrawn.  Similar  effects  are  obtained  if  a  magnet 
be  substituted  for  P,  or  if  in  the  first  experiment  the 
current  be  alternately  made  and  broken  in  P. 

8.    Ruhmkorff ' s  Induction  Coil  is  provided  with 
an  automatic  circuit-breaker,  consisting  of  an  electro- 

FIG.  237. 


Induction  Coil. 

magnet  whose  current  passes  through  a  spring  to 
which  the  armature  is  attached.  When  there  is  no 
current  this  spring  touches  a  "  contact  point "  which 
forms  part  of  a  circuit.  On  dipping  the  zinc  into  the 
acid  of  the  battery,  the  current  excites  the  electro- 
magnet. This  attracts  the  armature,  and  thus  removes 
the  spring  from  the  contact  point.  The  circuit  is  hence 
broken,  and  the  spring  draws  back  the  armature,  mak- 
ing contact  again.  By  this  device,  the  current  in  the 
primary  is  automatically  made  and  broken  with  great 
rapidity,  and  currents  alternately  in  opposite  directions 
are  induced  in  the  secondary.  The  intensity  of  these 
currents  may  be  greatly  increased  by  placing  a  bundle 
of  soft  iron  wires  in  the  axis  of  the  primary.  The  in- 
ductive effect  due  to  the  magnetizing  and  demagnet- 


TRANSFORMATION'S    OF    ENERGY.  339 

izing  of  the  iron  wires  by  the  making  and  breaking  of 
the  current  in  the  primary  is  thus  added  to  the  induc- 
tive effect  of  the  latter.  The  insulated  wire  of  the 
secondary  coil  is  long  and  fine,  sometimes  a  hundred 
miles  or  more  in  length.  The  electro-motive  force  of 
the  secondary  current  is  enormously  greater  than  that 
of  the  primary.*  Connected  with  the  poles  of  the  bat- 
tery is  a  condenser,  which  still  further  heightens  the 
effect  of  the  coil. 

The  induction  coil  is  used  for  many  purposes  re- 
quiring high  electro-motive  force,  and  is  usually 
more  reliable  than  any  machine  generating  electri- 
city by  friction.  Beautiful  effects  are  obtained  by 
passing  sparks  from  it  through  Q-eissler  tubes. 
These  are  made  of  glass,  and  contain  rarefied  gases  or 
vapors.  The  spark  when  passing  through  rarefied  hy- 
drogen assumes  a  brilliant  red  tint ;  through  nitrogen, 
a  gorgeous  purple.  With  the  proper  degree  of  rarefac- 
tion it  becomes  stratified  into  bands  across  the  tube. 

9.  The  Telephonef  is  an  instrument  for  utiliz- 
ing magneto-electric  currents  and  reproducing  speech 
by  their  aid.  Within  a  handle  of  vulcanite  is  a  per- 

*  The  largest  induction  coil  ever  constructed  was  made  for  Mr.  Spottis- 
woode,  an  English  physicist.  Its  secondary  coil  contained  280  miles  of 
wire,  wound  in  340,000  turns,  and  its  resistance  exceeded  100,000  ohms. 
When  worked  with  a  Grove  battery  of  30  cells,  it  gave  a  spark  42  inches 
long,  or  considerably  more  than  a  yard  in  length.  Coils  containing  50  miles 
of  wire  are  not  uncommon ;  1;hey  yield  sparks  a  foot  or  more  in  length.  A 
Leyden  jar  interposed  in  the  secondary  circuit  of  such  a  coil  is  charged 
and  discharged  so  rapidly  as  to  make  almost  a  continuous  sound. 

The  action  of  the  condenser  is  not  easily  explained  in  a  few  words  to 
the  elementary  student.  Consult  Thompson's  "  Lessons  in  Electricity  and 
Magnetism,"  pp.  363-365. 

t  A  telephone  in  parts,  ready  to  be  put  together  by  the  experimenter 
is  sold  by  the  apparatus  dealers.  A  simple  but  effective  instrument  can  be 


340 


ELECTRICITY. 


FIG.  238. 


manent  magnet  (Fig.  238),  around  one  pole,  N,  of 
which  is  an  insulated  coil,  C,  connected  with  the 
binding  posts  at  the  other  end.  A  thin  disk  of  soft 
iron,  BB,  is  fixed  across  near  the  encircled  magnet 
pole,  and  a  mouth-piece,  A,  serves  to  direct  the  sound 
of  the  voice  against  the  disk,  which  is  thus  made  to 

vibrate.  Distur- 
bances are  pro- 
duced in  the 
strength  of  the 
magnet,  and  cor- 
responding cur- 
rents traverse 
the  wire.  Pass- 
ing through  the 
coil  of  the  dis- 
tant telephone 
they  vary  the 
strength  of  its  magnet.  Minute  clicking  sounds  are 
produced  as  the  molecules  of  the  magnet  yield  to  these 
disturbances.  The  disk  re-enforces  these  like  a  sound- 
ing-board, and  gives  out  vibrations  to  the  air,  with 
such  rapidity  as  to  constitute  a  faithful  reproduction 
of  what  was  talked  into  the  transmitting  telephone.* 

made,  at  a  slight  expense,  by  a  pupil  with  ordinary  mechanical  ability. 
One  process,  with  illustrative  drawings,  is  given  in  the  "  Popular  Science 
Monthly,1'  March,  1878,  and  another  in  the  "  Scientific  American,"  Vol. 
XXXIX.,  No.  5.  In  Vol.  XXXIX.,  No.  16,  is  also  described  a  method  of 
constructing  a  microphone  ;  and  in  the  "Scientific  American  Supplement," 
No.  133,  is  an  account  of  a  home-made  phonograph.  These  numbers  can  be 
procured  by  any  newsdealer.  In  using  the  telephone,  two  instruments 
exactly  alike  are  employed.  One  is  held  to  the  mouth  of  the  speaker,  and 
the  other  to  the  ear  of  the  listener. 

*  It  should  be  observed  that  the  presence  of  a  disk  is  not  necessary  for 
the  perception  of  sound  from  the  receiving  telephone.    The  motion  is  prob- 


The  Telephone. 


TRANSFORMATIONS     OF     ENERGY.  341 

1C.  The  Microphone  is  a  modification  of  the  tele- 
phone transmitter.  It  consists  of  a  rod  of  gas  car- 
bon whose  ends  rest  loosely  in  cups  hollowed  out  of 
the  same  material,  and  these  in  turn  fixed  upon  a 
sounding-board.  The  current  from  a  battery  passes 
through  the  microphone  carbons  and  through  a  re- 
ceiving telephone.  If  the  sounding-board  be  made  to 
vibrate  in  the  least,  whether  by  sound-waves  or  by 
slight  mechanical  motion,  va- 

7  FIG.  239. 

riations  are  produced  in  the 
pressure  of  the  rod  against 
its  cups.  Two  pieces  of  carbon 
firmly  in  contact  conduct  elec- 
tricity moderately  well;  but 
if  the  pressure  between  them 
is  diminished,  the  resistance  is 
increased  and  the  current 
becomes  fainter.  The  micro- 
phone is  thus  the  last  refine- 

Microphone. 

ment  of  the  telegraph  com- 
bined with  the  telephone  receiver.  The  sound  of  the 
voice,  the  patter  of  a  fly's  foot  in  walking  over  the 
sounding-board,  or  the  gentlest  ticking  of  a  watch 
rested  upon  it,  are  thus  made  audible  in  a  telephone 
many  miles  away. 

ably  among  the  molecules  of  the  steel  magnet,  and  conducted  from  them 
to  the  disk  if  this  be  added. 

The  telephone  described  in  the  text  is  the  simplest  that  can  be  made. 
Many  improvements  have  been  effected  in  the  instrument.  The  transmit- 
ting telephone  is  now  generally,  made  in  such  manner  as  to  send  an  in- 
duced current,  like  that  of  the  Buhmkorff  coil,  through  the  line  wire  to 
the  receiver. 


342 


ELECTRICITY. 


11.  The  Magneto-electric  Machine. — The  produc- 
tion of  a  current  in  a  closed  circuit  by  moving  it  in 
the  field  of  a  magnet  (p.  336,  §  7)  is  realized  in  the 
construction  of  the  magneto-electric  machine.  This 
machine  consists  of  a  powerful  horseshoe  magnet  in 
front  of  which  a  pair  of  coils  is  made  to  rotate  (Fig. 
240).  This  pair  is  called  the  armature.  Each  coil 
contains  a  core  of  soft  iron,  which  acquires  and  then 

FIG.  240. 


The  Magneto-electric  Machina 

loses  magnetism,  as  it  approaches,  passes,  and  then 
recedes  from  a  pole  of  the  permanent  magnet.  In 
the  coil  these  rapid  variations  of  magnetic  strength 
produce  alternating  currents  whose  electro-motive 
force  is  determined  by  the  speed  of  rotation  and  the 
strength  of  the  magnetic  field.  The  two  ends  of  the 
coil  are  connected  with  insulated  plates  of  metal  on 
opposite  sides  of  the  axle.  On  each  of  these  a  con- 
ducting spring  presses,  which  carries  the  currents  to 
the  handles,  H.  This  arrangement,  called  a  commuta- 
tor, is  so  adapted  as  to  secure  but  one  direction  to 


TRANSFORMATIONS     OF     ENERGY.  343 

the  currents  in  the  main  wires.  On  taking  hold  of 
the  handles  while  the  shaft  is  rotated  rapidly,  a  series 
of  convulsive  shocks  is  experienced.* 

12.  The  Dynamo-electric  Machine.— For  the  gen- 
eration of  currents  to  be  employed  in  electric  light- 
ing it  is  necessary  that  they  shall  be  continuous 
rather  than  intermittent.  The  name  dynamo  is  ap- 
plied to  a  development  of  the  magneto-electric  ma- 
chine that  accomplishes  this  -result.  The  armature 
coils,  in  one  type  of  these  machines,  are  wound 
lengthwise  upon  a  drum  or  cylinder,  Fig.  241,  which 
is  revolved  between  the  poles  of  a  powerful  electro- 
magnet called  the  field-magnet.  On  this  drum  a 
large  number  of  coils  may  be  wound,  each  with  its 
own  pair  of  commutator  plates,  these  being  so  close 
together  that  the  interval  between  two  successive 


*  The  machine  represented  in  Eig.  240  is  known  as  Clarke's  machine, 
and  was  one  of  the  first  of  its  kind  invented.  Many  improvements  have 
been  subsequently  made.  In  1866,  Mr.  Wilde  discovered  that  if  the  in- 
duced current  be  passed  through  the  coil  of  an  electro-magnet,  the 
strength  it  produces  in  this  is  far  greater  than  that  qf  the  permanent 
magnet  employed.  An  additional  and  larger  armature  was  maae  to  rotate 
in  front  of  this  electro-magnet,  and  the  current  induced  in  it  was  made  to 
excite  a  second  and  still  larger  electro-magnet,  whose  armature  then  gen- 
erated currents  greatly  stronger  than  any  previously  known.  Such  a  ma- 
chine, driven  by  a  steam-engine  of  15-horse  power,  produces  an  electric 
light  dazzling  as  the  noonday  sun,  throwing  the  flame  of  the  street-lamps 
into  shade  at  a  quarter-mile  distance.  Its  heat  is  sufficient  to  fuse  a  J-inch 
bar  of  iron  fifteen  inches  long  or  7  feet  of  No.  6  iron  wire.—"  A  Yankee 
once  threw  the  industrial  world  of  Europe  into  a  wonderful  excitement  by 
announcing  a  new  theory  of  perpetual  motion  based  on  the  magneto-electric 
machine.  He  proposed  to  decompose  water  by  the  current  of  electricity ; 
then  burn  the  hydrogen  and  oxygen  thus  obtained.  In  this  way  he  would 
drive  a  small  steam-engine,  which,  in  turn,  would  keep  the  magneto-electric 
machine  in  motion.  This  would  certainly  be  a  splendid  discovery.  It 
would  be  a  steam-engine  which  would  prepare  its  own  fuel,  and,  in  addition, 
dispense  light  and  heat  to  all  around."— HELMHOLTZ. 


344 


ELECTRICITY. 


currents  is  imperceptible. 


FIG.  241. 


Drum  Armature  and  Four-part  Commu- 
tator. 

FIG.  242. 


Diagram  of  a  Series  Dynamo. 


In  Fig.  242,  the  end  of  the 
cylinder  and  of  the  group 
of  commutator  plates  are 
seen   between   the    large 
pole-pieces,  JVand  S,  of  the 
field-magnets.     The    cur- 
rent is  conducted  off  by 
the  springs  or  "brushes," 
and   passes   through  the 
coils  of  the  field-magnet 
before  reaching  the  main- 
line-wire.  The  pole-pieces 
never  quite  lose  their  mag- 
netism,   even    after    the 
machine  is  at  rest.    The 
energy    of    the    induced 
current  is  at  first  wholly 
absorbed  in  exciting  the 
field-magnet.  This  action, 
even    though   almost    in- 
finitely weak  at  first,  in- 
creases until  the  magnet 
is  as  strong  as  possible ; 
after  which  the  energy  is 
expended  in  doing  work 
on  the  main  circuit.* 

*  In  the  frontispiece  is  a  pic- 
ture of  the  Weston  Dynamo,  such 
as  is  used  for  producing  the  electric 
lights  on  the  great  bridge  between 
New  York  and  Brooklyn.  Each 
pole-piece  is  attached  to  two  coils 
which  are  so  wound  that  both  have 
the  same  effect  on  it.  The  end  of 


TRANSFORMATIONS     OF     ENERGY.  345 

13.  Electric    Motors. — The    action  of  a  dynamo 
is    reversible.      By    revolving    its    armature    we    ob- 
tain  a   current,  and  by  sending  a   current  through 
its  armature  we  cause  it  to  revolve.    The  same  ma- 
chine may,  therefore  be  used  either  as  a  dynamo  or  as 
a  motor.  The  dynamo  converts  the  mechanical  energy 
of  the  source  of  power  into  the  energy  of  the  electric 
current ;  the  motor  converts  the  energy  of  the  electric 
current  into  mechanical  energy.     On  this  principle 
depends  the  transmission  of  power. 

14.  The  Electric  Light. — According  to  the  method 
adopted  in  winding  the  armature,  a  dynamo-machine 
may  give  currents  of  high  or  of  low  electro-motive  force. 
Two  corresponding  kinds  of  lamp  are  used.    To  produce 
the  arc  light,  a  current  of  high  electro- motive  force  is 
sent  through  a  pair  of  carbon  rods,  which  are  then  drawn 
slightly  apart.    Particles  of  carbon  are  made  white-hot, 
and  even  turned  into  vapor,  which  is  thrown  from  one 
rod  to  the  other  in  the  direction  of  the  current.    The 
path  of  the  glowing  vapor  is  curved,  and  hence  this  is 
called  the  voltaic  arc.    It  is  the  most  brilliant  artificial 
light  known,  but  unsteady  because  the  arc  leaps  from 
side  to  side  as  the  carbons  become  wasted  away.    An 
automatic  regulator  is  employed  to  keep  them  at  the 
proper  distance  apart.    The  arc  light  is  excellent  for 
lighting  streets,  halls,  and  other  large  public  places. 
For  domestic  use,  the  incandescence  lamp  is  better. 

the  drum  armature  is  covered  with,  radiating  conductors,  which  connect 
the  coils  with  the  commutator  plates.  One  of  the  brushes  is  seen  pressing 
on  these  plates,  and  is  connected  with  the  insulated  wire  that  conducts 
the  current  away. 


346 


ELECTRICITY. 


In  this  a  current  of  low  electro-motive  force  passes 
through  a  filament  of  carbon  inclosed  in  a  globe 
from  which  most  of  the  air  has  been  withdrawn. 
The  filament  glows  with  a  soft  and  steady  light, 
which  is  much  inferior  in  brilliancy  to  the  arc  light. 
Although  only  J-OTTD-FOT}  of  the  air  remains  in  the 
globe,  the  filament  is  slowly  burned  away,  and  has 
to  be  replaced  with  a  new  one. 

15.  Thermo-electricity. — In  the  electric  lamp  the 
energy  of  a  current  is  changed  into  heat  and  light. 
Conversely,  heat  and  light  (radiant  energy)  may  be 
changed  into  electric  energy.  If  the  end  of  an  iron 
wire  be  connected  with  that  of  a  copper  or  German- 
silver  wire,  the  other  ends  being  attached  to  a  gal- 
vanometer, the  needle  will  swing  aside  when  the 
joined  ends  are  heated.  This  effect  is  increased  if 
the  junction  be  made  with  bismuth  and  antimony. 
A  current  flows  at  the  junction  from  bismuth  to 
antimony,  thence  through  the  galvanometer  back  to 
the  bismuth. 

A  THERMO-ELECTRIC  PILE  consists  of  alternate  bars 

of  antimony  and  bismuth 
soldered  together,  as  shown 
in  Fig.  243.  When  mount- 
ed for  use,  the  couples  are 
insulated  from  each  other 
and  inclosed  in  a  copper 
frame,  P.    If  both  faces  of  the  pile 
are  equally  heated,   there  is  no  cur- 
rent.   The  least  variation  of  temper- 
ature, however,  between  the  two  is  indicated  by  the 


Fig.  243. 


Fig.  244. 


Thermopile. 


TRANSFORMATIONS     OF     ENERGY.  347 

flow  of  electricity.  Wires  from  a,  the  positive  pole, 
and  6,  the  negative,  connect  the  pile  with  the  gal- 
vanometer. This  furnishes  a  test  of  change  of  tem- 
perature. A  fly  walking  over  the  face  of  the  pile  by 
its  warmth  will  move  the  needle,  if  the  galvanometer 
be  very  delicate.  When  skillfully  used,  the  thermo- 
pile serves  as  a  very  sensitive  thermometer. 

THE  BOLOMETER  is  an  instrument  devised  for  the 
detection  of  very  faint  variations  of  temperature.  A 
platinum  or  iron  wire  opposes  much  more  resistance 
to  the  passage  of  an  electric  current  when  hot  than 
when  cold.  The  current  is  made  to  divide  between 
two  conductors.  These  are  connected  by  a  cross- 
wire,  or  "  bridge,"  with  a  galvanometer  interposed. 
If  the  current  in  the  two  branches  be  equal,  the  gal- 
vanometer is  not  affected ;  but,  if  unequal,  a  cross- 
current deflects  the  galvanometer  needle.  By  heat- 
ing one  branch  slightly  the  balance  is  disturbed,  and 
the  difference  of  temperature  is  read  in  the  deflection 
of  the  needle.* 

16.  Animal  Electricity. — The  human  body  is  often 
electrified.  Many  animals,  especially  when  angry  or 
otherwise  excited,  give  evidence  of  being  electrified. 

*  This  instrument  was  invented  by  Professor  Langley  at  the  Alleghany 
Observatory  near  Pittsburg.  It  was  used  in  examining  the  invisible  parts  of 
the  solar  spectrum,  where  lines  and  bands  were  discovered  whose  presence 
could  not  be  detected  with  the  most  delicate  thermopile.  The  invisible 
part  of  the  spectrum  was  thus  found  to  be  much  more  extensive  than  the 
visible  part,  while  the  most  intense  heat  as  well  as  light  is  found  in  the 
region  colored  greenish-yellow.  The  bolometer  is  capable  of  revealing  a 
change  of  temperature  of  .00001°  C.  Professor  Langley  has  discovered  by 
this  means  that  the  highest  temperature  of  the  moon  scarcely,  if  at  all, 
exceeds  that  of  the  human  body,  and  that  the  temperature  of  outer  space 
is  nearly  as  low  as  the  absolute  zero  of  temperature,  —  273°  C. 


348  ELECTRICITY. 

Certain  fish  have  the  property  of  giving,  when 
touched,  a  shock  like  that  from  a  Leyden  jar.  The 
torpedo  and  the  electrical  eel  are  most  noted.  The 
former  is  a  native  of  the  Mediterranean,  and  its 
shock  was  anciently  prized  as  a  cure  for  various  dis- 
eases. The  latter  is  abundant  in  certain  South 
American  waters.  A  specimen  of  this  fish,  forty 
inches  in  length,  was  estimated  by  Faraday  to  emit 
a  spark  equal  to  the  discharge  of  a  battery  of  fifteen 
Leyden  jars  of  3,500  square  inches  surface. 


SUMMARY. 

ELECTRICITY  is  a  form  of  energy  that  may  be  manifested  as 
an  accompaniment  of  friction,  of  chemical  action,  of  the  motion 
of  magnets,  of  variations  in  temperature,  or  of  animal  excite- 
ment. It  exhibits  a  certain  kind  of  duality  in  its  effects,  and 
hence  the  names  positive  and  negative  electricity  are  used  to 
express  the  contrast.  Many  considerations  point  to  the  conclu- 
sion that  the  molecules  of  a  charged  body  are  in  a  condition  of 
strain.  This  condition  can  be  communicated  by  induction 
through  a  "dielectric,"  which  itself  becomes  strained  while  thus 
acting  as  a  medium.  By  taking  advantage  of  a  proper  dielec- 
tric, such  as  glass,  electrical  energy  may  be  stored  up  for  subse- 
quent use,  as  in  the  Leyden  jar. 

Voltaic  electricity  has  its  origin  in  chemical  action,  or  in 
contact  of  different  metals,  or  in  both.  The  essentials  of  an 
ordinary  battery  for  its  development  are  two  substances,  which 
are  unequally  affected  by  a  chemical  agent.  One  of  these  is  at 
higher  potential  than  the  other,  and  neutralization  is  effected  by 
the  passage  of  a  current,  continually  renewed,  from  the  body 
at  high  potential,  through  the  best  conductor,  to  the  body  at 
low  potential.  This  difference  of  potential,  however,  is  very 
slight  in  comparison  with  that  developed  by  friction  and  induc- 
tion, as  in  the  Holtz  or  Voss  machine.  Voltaic  electricity  is 
more  manageable,  more  reliable,  more  convenient,  more  gener- 


HISTORICAL     SKETCH.  349 

ally  available  than  frictional  electricity.  Electricity  may  be 
transformed,  under  appropriate  conditions,  into  mechanical 
motion,  magnetism,  sound,  heat,  or  light.  Among  its  most  im- 
portant applications  to  the  purposes  of  practical  life  are  the 
telegraph,  the  electrotype,  the  telephone,  distribution  of  power, 
and  the  electric  light. 

HISTORICAL     SKETCH. 

THALES  (6th  cent.  B.  c.),  one  of  the  seven  wise  men,  knew 
that  when  amber  is  rubbed  with  silk  it  will  attract  light  bodies, 
as  straw,  leaves,  etc.  This  property  was  considered  so  marvel- 
ous that  amber  was  supposed  to  possess  a  soul.  From  the 
Greek  name  of  the  substance  (elektron)  our  word  electricity  is 
derived.  This  simple  phenomenon  constituted  all  that  was 
known  until  the  16th  century,  when  William  Gilbert,  physician 
to  Queen  Elizabeth,  made  many  valuable  experiments.  He  dis- 
covered that  amber  was  by  no  means  the  only  substance  which 
can  exhibit  electrical  manifestations  when  rubbed,  and  he  exam- 
ined into  the  conditions  favorable  to  electrical  phenomena. 
Among  the  most  important  of  these  he  found  to  be  the  dryness 
of  the  atmosphere.  Francis  Hawksbee  called  attention  to  the 
resemblance  between  the  electric  spark  and  lightning,  and  in- 
vented an  electric  machine,  in  which  the  hands  were  used  as 
rubbers.  Stephen  Gray,  in  the  18th  century,  discovered  the  dif- 
ference between  conductors  and  non-conductors,  that  an  electric 
charge  is  at  the  surface,  and  that  the  human  body  can  be  elec- 
trified. Dufay  discovered  that  there  are  two  manifestations  of 
electricity,  which  he  called  vitreous  and  resinous,  and  considered 
them  to  be  fluids.  Kinnersley,  the  friend  and  associate  of 
Franklin,  recognized  that  these  two  electricities  were  nothing 
else  than  what  Franklin  had  already  called  positive  and  nega- 
tive charges.  The  Leyden  jar  was  invented  in  1745,  probably 
by  several  persons  about  the  same  time ;  it  was  first  exhibited 
and  used  in  experiment  by  Muschenbroeck,  at  Leyden,  in  Hol- 
land. By  the  use  of  it  students  of  electricity  were  able  to 
gather  the  mysterious  "virtue"  or  "effluvia"  in  much  larger 
quantities,  and  to  produce  effects  never  imagined  before,  such 
as  the  firing  of  gunpowder.  Experiments  were  made  about  this 


350  ELECTRICITY. 

time  to  ascertain  the  rate  of  transmission  of  electricity  from  a 
Leyden  jar  through  a  metallic  conductor.  A  wire  more  than 
two  miles  long  was  employed ;  through  this  the  discharge  ap- 
peared to  be  absolutely  instantaneous. 

In  1749,  Benjamin  Franklin  wrote  from  Philadelphia  to 
Peter  Collinson  at  London,  as  follows : 

"Chagrined  a  little  that  we  have  hitherto  been  able  to 
produce  nothing  in  this  way  of  use  to  mankind,  and  the  hot 
weather  coming  on,  when  electrical  experiments  are  .not  so 
agreeable,  it  is  proposed  to  put  an  end  to  them  for  this  season, 
somewhat  humorously,  in  a  party  of  pleasure  on  the  banks  of 
the  SkuylkiL  Spirits,  at  the  same  time,  are  to  be  fired  by  a 
spark  sent  from  side  to  side  through  the  river,  without  any 
other  conductor  than  the  waber ;  an  experiment  which  we  some 
time  since  performed,  to  the  amazement  of  many.  A  turkey  is 
to  be  killed  for  our  dinner  by  the  electrical  shock,  and  roasted 
by  the  electrical  jack  before  a  fire  kindled  by  the  electrical 
bottle  (Leyden  jar);  when  the  healths  of  all  the  famous  elec- 
tricians in  England,  Holland,  France,  and  Germany  are  to  be 
drank  in  electrified  bumpers,  under  the  discharge  of  guns  from 
the  electrical  battery." 

About  1752,  Franklin  proved  the  identity  of  lightning  and 
frictional  electricity  by  means  of  a  kite  made  of  a  silk  hand- 
kerchief and  with  a  pointed  wire  at  the  top.  He  elevated  this 
during  a  thunder-storm,  tying  at  the  end  of  the  hemp  string  a 
key,  and  then  insulating  the  whole  by  fastening  it  to  a  post 
with  a  long  piece  of  silk  lace.  On  presenting  his  knuckles  to 
the  key,  he  obtained  a  spark.  He  afterward  charged  a  Leyden 
jar,  and  performed  other  electrical  experiments  in  this  way. 
These  attempts  were  attended  with  very  great  danger.  Prof. 
Kichman,  of  St.  Petersburg,  drew  in  this  manner  from  the 
clouds  a  ball  of  blue  fire  as  large  as  a  man's  fist  which  struck 
him  lifeless.  Shortly  after  the  famous  experiments  of  Franklin, 
the  Frenchman,  Coulomb,  established  the  law  of  electric  attrac- 
tion and  repulsion,  showing  that  it  was  the  same  as  that  of 
gravitation,  light,  and  heat,  the  law  of  inverse  squares. 

In  the  year  1790,  Galvani  was  engaged  in  some  experiments 
on  animal  electricity.  For  this  purpose  he  used  frogs'  legs  as 
electroscopes.  He  had  hung  several  of  these  upon  copper  hooks 


HISTORICAL     SKETCH.  351 

from  the  iron  railing  of  the  balcony,  in  order  to  see  what  effect 
the  atmospheric  electricity  might  have  upon  them.  He  noticed, 
to  his  surprise,  that  when  the  wind  blew  them  against  the  iron 
supports,  the  legs  were  convulsed  as  if  in  pain.  After  repeated 
experiments,  Q-alvani  concluded  that  this  effect  was  produced 
by  what  he  termed  animal  electricity,  that  this  electricity  is 
different  from  that  caused  by  friction,  and  that  he  had  discov- 
ered the  agent  by  which  the  will  controls  the  muscles.  Volta 
rejected  the  idea  of  animal  electricity,  and  held  that  the  contact 
of  dissimilar  metals  was  the  source  of  the  electricity,  while  the 
frog  was  "only  a  moist  conductor,  and  for  that  pur- 
pose was  not  as  good  as  a  wet  rag."  He  applied  this 
view  to  the  construction  of  "Volta's  pile,"  which  is 
composed  of  plates  of  zinc  and  copper,  between  which 
are  laid  pieces  of  flannel  moistened  with  an  acid  or  a 
saline  solution  (Fig.  245).  This  theory  is  substantially 
the  one  held  at  the  present  time,  though  we  now  know 
that  there  must  be  chemical  action  to  continue  the 
supply. 

From  the  earliest  times  in  which  the  knowledge  of 
electricity  began  to  be  definite,  impostors  and  half- 
educated  people  circulated  marvelous  stories  about  its  value  as 
a  panacea  for  all  kinds  of  disease.  Many  supposed  that  deaf- 
ness and  dimness  of  sight  might  be  cured  by  the  use  of  the 
electric  spark.  Franklin  remarked  of  this,  "it  will  be  well  if 
perfect  blindness  be  not  the  consequence  of  the  experiment." 
In  the  hands  of  experienced  physicians  electricity  has  been  used 
with  good  effect,  but  to-day,  as  in  Franklin's  time,  the  name 
often  serves  as  a  cloak  for  ignorance  or  trickery. 

Electricity  and  magnetism  were  studied  as  distinct  branches 
until  1820,  when  Oersted  of  Copenhagen  discovered  the  phe- 
nomenon shown  in  Fig.  224.  This  was  published  every-where, 
and  excited  the  deepest  interest  of  scientific  men.  In  the  fruit- 
ful mind  of  Ampere  the  experiment  bore  abundant  fruit.  He 
discovered  that  two  parallel  wires  conveying  an  electric  cur- 
rent in  the  same  direction  attract  each  other,  and  when  in 
opposite  directions,  repel  each  other.  From  this  he  generalized 
the  entire  subject.  Prof.  Henry  next  exhibited  the  wonderful 
power  of  the  electro-magnet,  and  invented  the  electro-magnetic 


352  ELECTRICITY. 

engine.  Scientific  men  in  all  parts  of  the  world  were  now 
gathering  the  material  necessary  for  the  invention  of  the  elec- 
tric telegraph.  It  fell  to  Samuel  F.  B.  Morse  to  make  this 
knowledge  practical,  and  in  1837  he  exhibited  in  New  York  a 
working  instrument.  An  experimental  line  between  Washing- 
ton and  Baltimore  was  completed  in  1844,  and,  on  May  27th  of 
that  year,  was  sent  the  first  message  ever  forwarded  by  a  re- 
cording telegraph. 

Consult  Maxwell's  "Electricity  and  Magnetism";  Tyndall's 
"Lessons  in  Electricity";  "Faraday's  "Lectures  on  the  Phys- 
ical Forces"  and  "Researches  in  Electricity";  Noad's  "Manual 
of  Electricity";  Art.  on  the  Microphone,  in  "Scribner's  Monthly," 
Vol.  XVI.,  p.  600  ;  Prescott's  "  The  Speaking  Telephone,  Talk- 
ing Phonograph,"  etc.;  Foster's  "Electrical  Measurements,"  in 
"Science  Lectures  at  South  Kensington,"  Vol.  I.,  p.  264  ;  Thom- 
son's "Papers  on  Electrostatics  and  Magnetism";  Guillemin's 
"The  Forces  of  Nature"  and  "The  Applications  of  Physical 
Forces";  "American  Cyclopedia,"  Articles  on  Electricity,  Mag- 
netism, Electro-magnetism,  etc.;  Smith's  "Manual  of  Teleg- 
raphy"; Jones'  "  Historical  Sketch  of  Electric  Telegraph";  Watts' 
"Electro-metallurgy";  "Barnes'  Hundred  Years  of  American 
Independence,"  Sec.  on  Morse,  p.  442 ;  "  Fourteen  Weeks  in 
Zoology,"  Sec.  on  Torpedo,  p.  186;  Gordon's  "  Electricity  and 
Magnetism";  Hospitalier's  "  Modern  Applications  of  Electricity" 
(specially  commended  for  latest  discoveries);  Urbanitzki's  "  Elec- 
tricity in  the  Service  of  Man";  Mendenhall's  "A  Century  of 
Electricity";  Thompson's  "Dynamo-electric  Machinery";  Dan- 
iell's  "Principles  of  Physics,"  and  Anthony  and  Brackett's 
"Text-book  of  Physics." 


CONCLUSION. 

"Science  is  a  psalm  and  a  prayer."— PARKER, 

NOWHERE  in  nature  do  we  find  chance.  Every 
event  is  governed  by  fixed  laws.  If  we  would  ac- 
complish any  result  or  perform  any  experiment,  we 
must  come  into  exact  harmony  with  the  universal 
system.  If  we  deviate  from  the  line  of  law  by  a 
hair's  breadth,  we  fail.  These  laws  have  been  in 
operation  since  the  earliest  beginnings  of  the  devel- 
opment of  our  world,  and  all  the  discoveries  of  sci- 
ence prove  them  to  extend  to  the  most  distant  star 
in  space.  A  child  of  to-day  amuses  itself  with  cast- 
ing a  stone  into  the  brook  and  watching  the  widen- 
ing curves ;  little  children  of  ten  thousand  years  ago 
may  have  done  the  same.  A  law  of  nature  has  no 
force  of  itself ;  it  is  but  the  manner  in  which  force 
acts. 

We  can  not  create  force.  We  find  it  every-where 
in  Nature ;  so  that  matter  is  not  dumb,  but  full  of 
inherent  energy.  A  tiny  drop  of  dew  sparkling  on 
a  spire  of  grass  is  instinct  with  power :  Gravity 
draws  it  to  the  earth ;  Chemical  Affinity  binds  to- 
gether the  atoms  of  hydrogen  and  oxygen ;  Cohesion 
holds  the  molecules  of  water,  and  gathers  the  drop 
into  a  globe ;  Heat  keeps  it  in  the  liquid  form ;  Ad- 
hesion causes  it  to  cling  to  the  leaf.  If  the  water 


354  CONCLUSION. 

be  decomposed,  Electricity  will  be  set  free ;  and  from 
this,  Heat,  Light,  Magnetism,  and  Motion  can  be 
produced.  Thus  the  commonest  object  becomes  full 
of  fascination  to  the  scientific  mind,  since  in  it  reside 
the  mysterious  forces  of  Nature. 

These  various  forces  can  be  classified  either  as 
attractive  or  repellent.  Under  their  influence  the 
atoms  or  molecules  resemble  little  magnets  with 
positive  and  negative  poles.  They  approach  or  re- 
cede from  one  another,  and  so  tend  to  arrange 
themselves  according  to  some  definite  plan.  "The 
atoms  march  in  time,  moving  to  the  music  of  law." 
A  crystal  is  but  a  specimen  of  "molecular  archi- 
tecture" built  up  by  the  forces  with  which  matter 
is  endowed.  Forces  continually  ebb  and  flow,  but 
the  sum  of  energy  through  the  universe  remains 
the  same.  In  time  all  the  possible  changes  may  be 
rung,  and  the  various  forms  of  energy  subside  into 
one  uniformly-diffused  heat-quiver,  but  in  that  will 
exist  the  representation  of  all  the  forces  which  now 
animate  creation. 


XI. 

APPENDIX. 


APPENDIX. 


QUESTIONS. 

THE  following  questions  are  those  which  the  author  has 
used  in  his  classes,  both  as  a  daily  review  and  for  examina- 
tion. A  standing  question,  which  has  followed  every  other 
question,  has  been:  "Can  you  illustrate  this?"  Without,  there- 
fore, a  particular  request,  the  pupil  has  been  accustomed  to 
give  as  many  practical  examples  as  he  could,  whenever  he  has 
made  any  statement  or  given  any  definition. 

I.  Introduction.  —  Define  matter.  A  body.  A  substance. 
Name  and  define  the  two  kinds  of  properties  which  belong  to 
each  substance.  State  the  suppositions  of  the  Atomic  Theory. 
What  is  a  molecule  ?  An  atom  ? 

Describe  the  two  kinds  of  change  to  which  matter  may  be 
subjected.  What  is  the  principal  distinction  between  Physics 
and  Cheirfistry?  Mention  some  phenomena  which  belong  to 
each.  Why  are  these  branches  intimately  related? 

Name  the  general  properties  of  matter.  Define  magnitude 
Size.  Distinguish  between  size  and  mass.  Why  are  feathers 
light  and  lead  heavy?  Why  is  it  necessary  to  have  a  standard 
of  measure?  What  are  the  French  and  English  standards? 
Give  the  history  of  the  English  standard.  Is  the  American 
yard  an  exact  copy  of  the  English?  Give  an  account  of  the 
French  system.  By  what  name  is  this  system  commonly 
known?  Is  either  of  these  systems  founded  on  a  natural  stand- 
ard? Why  is  it  desirable  to  have  such  a  standard? 

Define  Impenetrability.  Give  some  apparent  exceptions, 
and  explain  them.  Define  Divisibility.  Is  there  any  limit  to 
the  divisibility  of  matter?  Define  Porosity.  Is  the  word  porous 


358  APPENDIX. 

used  here  in  its  common  acceptation?  What  practical  use  is 
made  in  the  arts  of  the  property  of  porosity?  Describe  the 
experiment  of  the  Florence  academicians. 

Define  Inertia.  Does  a  ball,  when  thrown,  stop  itself?  Why 
is  it  difficult  to  start  a  heavy  wagon?  Why  is  it  dangerous  to 
jump  from  the  cars  when  in  motion?  (Compare  First  Law  of 
Motion.)  Define  Indestructibility.  Has  the  earth  at  all  times 
contained  the  same  quantity  of  matter  that  it  does  now? 

Name  the  specific  properties  of  matter.  Define  Ductility. 
How  is  iron  wire  made?  Platinum  wire?  Gilt  wire?  Define 
Malleability. 

Describe  the  manufacture  of  gold-leaf.  Is  copper  malleable? 
Define  Tenacity.  Name  and  define  the  three  kinds  of  Elas- 
ticity. Illustrate  the  elasticity  of  compression  as  seen  in  solids. 
In  liquids.  In  gases.  What  is  said  about  the  relative  com- 
pressibility of  liquids  and  gases? 

Illustrate  the  elasticity  of  expansion  as  seen  in  solids,  liquids, 
and  gases.  Define  Elasticity  of  Torsion.  What  is  a  Torsion 
balance?  Define  Hardness.  Does  this  property  depend  on 
density?  Define  Density.  Define  Brittleness.  Is  a  hard  body 
necessarily  brittle?  Name  a  brittle  and  a  hard  body. 

II.  Motion  and  Force.— Define  motion,  absolute  and  relative. 
Best.  Velocity.  Force.  What  are  the  resistances  to  motion? 
Tell  what  you  can  about  friction.  Why  does  oil  diminish  fric- 
tion? What  uses  has  friction?  What  law  governs* the  resist- 
ance of  air  or  water?  Define  Momentum. 

Show  that  motion  is  not  imparted  instantaneously.  State 
the  three  laws  of  motion  and  the  proof  of  each.  If  a  ball  be 
fired  into  the  air  when  a  horizontal  wind  is  blowing,  will  it  rise 
as  high  as  if  the  air  were  still?  Describe  the  experiments  with 
the  collision  balls.  Give  practical  illustrations  of  action  and 
reaction.  If  a  bird  could  live,  could  it  fly  in  a  vacuum?  De- 
fine compound  motion. 

Define  the  so-called  "  parallelogram  of  forces."  The  result- 
ant. How  can  the  resultant  of  two  or  more  forces  be  found? 
Name  some  practical  illustrations  of  compound  motion.  What 
is  the  "resolution  of  forces"? 

Show  how  one  vessel    can    sail    south    and    another    north, 


QUESTIONS.  359 

driven  by  the  same  westerly  wind.  Explain  how  a  kite  is 
raised.  Explain  the  "split-shot"  in  croquet. 

Explain  the  towing  of  a  canal-boat.  Describe  how  motion 
in  a  curve  and  circular  motion  are  produced.  Explain  the  cen- 
tripetal and  centrifugal  forces. 

Show  when  the  centrifugal  force  becomes  strong  enough  to 
overcome  the  force  of  Cohesion.  Of  Adhesion.  Of  Gravity. 
Apply  the  principle  of  circular  motion  to  the  revolution  of  the 
earth  about  the  sun.  What  effect  does  the  revolution  of  the 
earth  on  its  axis  have  upon  all  bodies  on  the  surface? 

What  would  be  the  effect  if  the  rotation  were  to  cease?  De- 
scribe the  action  of  the  centrifugal  force  on  a  hoop  rapidly  re- 
volved on  its  axis.  Define  reflected  motion.  Give  its  law. 

What  is  Energy,  in  the  Physical  sense  of  the  word?  To 
what  ts  it  proportional?  Name  and  define  the  two  forms  of 
energy.  How  may  one  form  be  changed  into  the  other? 

What  is  the  law  of  the  Conservation  of  Energy?  What  did 
Faraday  say  with  regard  to  this  law? 

III.  Attraction.  1.  MOLECULAR  FORCES. — Define  a  molecular 
force.  What  two  opposing  forces  act  between  the  molecules  of 
matter?  How  is  this  shown?  What  is  the  repellent  force? 
Name  the  attractive  forces.  Which  of  these  belong  to  Physics? 

1.  Cohesion.— Define.    What  are  the  three  states  of  matter? 
Define.      How  can  a  body  be   changed  from  one  state  to  an- 
other? 

Show  that  cohesion  acts  only  at  insensible  distances.  Ex- 
plain the  process  of  welding.  Why  do  drops  of  dew,  etc.,  take 
a  globular  form  ?  Why  do  not  all  bodies  have  this  form  ?  Illus- 
trate the  tendency  of  matter  to  a  crystalline  structure. 

Has  each  substance  its  own  form?  Why  is  not  cast-iron 
crystalline?  Why  do  cannon  become  brittle  after  long  use? 

Describe  the  process  of  tempering  and  annealing.  Explain 
the  Rupert's  Drop.  How  is  glassware  annealed? 

2.  Adhesion. — Define.     What  is  the  theory  of  filtering  through 
charcoal  ?    Of  what  use  is  soap  in  making  bubbles  ?    Define  Cap- 
illary Attraction.     Why  will  water  rise  in  a  glass  tube,  while 
"mercury  will  be  depressed?    Is  a  tube  necessary  to  show  capil- 
lary attraction?    What  is  the  law  of  the  rise  in  tubes? 


360  APPENDIX. 

Give  practical  illustrations  of  capillary  action.  Why  will 
not  old  cloth  shrink  as  well  as  new,  when  washed  ?  What  is  the 
cause  of  solution?  Why  is  the  process  hastened  by  pulveriz- 
ing? 

Tell  what  you  can  about  gases  dissolving  in  water.  Why 
does  the  gas  escape  from  soda-water  as  soon  as  drawn?  Why 
do  pressure  and  cold  favor  the  solution  of  a  gas  ?  Describe  the 
diffusion  of  liquids.  Of  gases. 

Describe  the  osmose  of  liquids.  Of  gases.  Why  do  rose- 
balloons  lose  their  buoyancy?  What  is  the  difference  between 
the  osmose  and  the  diffusion  of  gases? 

2.  GRAVITATION. — How  does  Gravitation  differ  from  Cohesion 
and  Adhesion?  What  is  the  law  of  gravitation?  Why  does  a 
stone  fall  to  the  ground?  Will  a  plumb-line  near  a  mountain 
hang  perpendicularly  ?  Why  do  the  bubbles  in  a  cup  «of  tea 
gather  on  the  side  ?  How  is  the  earth  kept  in  its  place  ?  Define 
Gravitation.  Gravity.  Weight. 

State  the  three  laws  of  weight.  What  is  a  vertical  or  plumb- 
line? 

Describe  the  "  guinea-and-f eather  experiment."  What  does 
it  prove?  Describe  Atwood's  machine.  Deduce  the  formulas 
for  falling  bodies. 

How  can  the  time  of  a  falling  body  be  used  for  determining 
the  depth  of  a  well?  How  does  gravity  act  upon  a  body 
thrown  upward?  What  velocity  must  be  given  to  a  ball  to  ele- 
vate it  to  any  point?  How  high  will  it  rise  in  a  given  time? 
When  it  falls,  with  what  force  will  it  strike  the  ground  ?  Define 
the  Center  of  Gravity.  The  line  of  direction.  The  three  states 
of  equilibrium. 

How  may  the  center  of  gravity  be  found  ?  Give  the  general 
principles  of  the  center  of  gravity.  Describe  the  leaning  tower 
of  Pisa.  State  some  physiological  applications  of  the  center  of 
gravity.  Why  do  fat  people  generally  walk  so  erect? 

Define  the  Pendulum.  Arc.  Amplitude.  What  are  isochro- 
nous vibrations?  State  the  three  laws  of  the  pendulum.  Who 
discovered  the  first  law?  What  is  the  center  of  oscillation? 
How  is  it  found?  What  is  the  center  of  percussion? 

Describe  the  pendulum  of  a  clock.  How  is  a  clock  regu- 
lated? Does  it  gain  or  lose  time  in  winter?  Describe  the 


QUESTIONS.  361 

gridiron  pendulum.  The  mercurial  pendulum.  Name  the 
various  uses  of  the  pendulum.  Describe  Foucault's  experi- 
ment. 

IV.  The   Elements  of  Machines. — Name  and  define  the  ele- 
ments of  machinery.      Do  the  "  powers,"  so  called,  produce  en- 
ergy?    What   is    the    law    of   mechanics?     Illustrate   the   law. 
What  is   a  lever?     Describe  the  three  classes  of  levers.     The 
law  of  equilibrium. 

What  is  the  advantage  peculiar  to  each  class?  Describe  the 
steelyard  as  a  lever.  What  effect  does  it  have  to  reverse  the 
steelyard?  Describe  the  arm  as  a  lever.  (See  "Hygienic 
Physiology,"  p.  34.)  Would  a  lever  of  the  first  class  answer 
the  purpose  of  the  arm?  Describe  the  compound  lever. 

Describe  the  hay  scale.  The  wheel  and  axle.  Its  law  of 
equilibrium.  Describe  a  system  of  wheel-work.  At  which  arm 
of  the  lever  is  the  P  applied? 

Describe  the  various  uses  of  the  inclined  plane.  Its  law  of 
equilibrium.  What  velocity  does  a  body  acquire  in  rolling  down 
an  inclined  plane?  Give  illustrations. 

Describe  the  screw.  Its  uses.  Its  law  of  equilibrium. 
How  may  its  power  be  increased?  What  limit  is  there? 
Describe  the  wedge.  Its  uses.  Its  law  of  equilibrium.  How 
does  it  differ  from  that  of  the  other  powers?  Describe  the 
pulley.  The  use  of  fixed  pulleys.  Is  there  any  gain  of  P  in 
a  fixed  pulley? 

What  is  the  use  of  a  movable  pulley?  Describe  a  movable 
pulley  as  a  lever.  Give  the  general  law  of  equilibrium  in  a 
combination  of  pulleys.  What  are  cumulative  contrivances*/ 
Is  perpetual  motion  possible?  Why? 

V.  Pressure  of  Liquids  and   Gases.      1.   HYDROSTATICS.— De- 
fine.    What  liquid  is  taken  as  the  type?     What  is  the  first  law 
of  liquids?     Explain.      Illustrate  the  transmission    of  pressure 
by  water.      Show   how   water  is  used  as  a  mechanical  power. 
Describe  the  hydrostatic  press.      Give  its  law  of  equilibrium. 

What  are  the  uses  of  this  press?  What  pressure  is  sus- 
tained by  the  lower  part  of  a  vessel  of  water,  when  acted  on 
by  gravity  alone?  How  does  this  pressure  act?  State  the  four 
laws  which  depend  on  this  principle,  and  illustrate  them."  What 


362  APPENDIX. 

is  the  weight  of  a  cubic  foot  of  sea  water?  Fresh  water? 
What  is  the  pressure  at  two  feet?  Give  illustrations  of  the 
pressure  at  great  depths.  Describe  the  hydrostatic  bellows.  Its 
law  of  equilibrium.  What  is  the  "hydrostatic  paradox"?  Give 
illustrations.  Give  the  principle  of  fountains.  How  high  will 
the  water  rise?  How  do  modern  engineers  carry  water  across 
a  river?  Did  the  ancients  understand  this  principle?  Give 
the  theory  of  the  Artesian  well,  and  of  ordinary  wells  and 
springs. 

Give  the  rule  for  finding  the  pressure  on  the  bottom  of  a 
vessel.  On  the  side.  Define  the  water  level.  Is  the  surface 
of  water  horizontal?  If  it  were,  what  part  of  an  approaching 
ship  would  we  see  first  ?  Describe  the  spirit-level.  Define  spe- 
cific gravity.  What  is  the  standard  for  solids  and  liquids?  For 
gases?  Explain  the  buoyant  force  of  liquids. 

What  is  Archimedes'  law?  Describe  the  "  cylinder  -  and- 
bucket  experiment."  What  does  it  prove?  Give  the  method  of 
finding  the  specific  gravity  of  a  solid.  A  liquid. 

Is  it  necessary  to  use  a  specific  gravity  flask  holding  just 
1,000  oz.,  or  would  any  size  answer?  Suppose  the  solid  is 
lighter  than  water  and  will  not  sink,  what  can  you  do?  Ex- 
plain the  hydrometer.  How  can  you  find  the  weight  of  a  given 
volume  of  any  substance?  The  volume  of  any  given  weight? 
The  exact  volume  of  a  body?  Illustrate  the  action  of  dense 
liquids  on  floating  bodies.  Why  will  an  iron  ship  float  on 
water?  Where  is  the  center  of  gravity  in  a  floating  body? 
How  do  fish  sink  at  pleasure? 

2.  HYDRODYNAMICS. — Define.  To  what  is  the  velocity  of  a  jet 
equal?  How  is  the  velocity  found?  Give  the  rule  for  finding 
the  quantity  of  water  which  can  be  discharged  from  a  jet  in  a 
given  time.  What  is  the  effect  of  tubes?  Tell  something  of 
the  flow  of  water  in  rivers. 

Name  and  describe  the  different  kinds  of  water-wheels. 
Which  is  the  most  valuable  form?  Describe  Barker's  Mill. 
How  are  waves  produced?  Explain  the  real  motion  of  the 
water.  How  does  the  motion  of  the  whole  wave  differ  from 
that  of  each  particle?  How  is  the  character  of  waves  modified 
near  the  shore?  What  is  the  extreme  height  of  "mountain 
waves  "  ?  Define  like  phases.  Unlike  phases.  A  wave-length. 


QUESTIONS.  363 

What  is  the  effect  if  two  waves  with  like  phases  coincide? 
With  unlike  phases?  What  is  this  termed? 

3.  PNEUMATICS. — Define.  What  principles  are  common  to 
liquids  and  gases?  What  gas  is  taken  as  the  type?  Describe 
the  air-pump.  Can  a  perfect  vacuum  be  obtained  in  this  way? 
What  is  the  condenser?  Its  use?  Prove  that  the  air  has  weight. 

Show  its  elasticity  and  compressibility.  Describe  the  bottle- 
imps.  What  principles  do  they  illustrate?  Show  the  expansi- 
bility of  the  air. 

Describe  the  experiments  with  the  hand-glass.  The  principle 
of  Hiero's  fountain.  The  Magdeburg  hemispheres.  What^do 
they  prove?  Show  the  upward  pressure  of  the  air. 

The  buoyant  force  of  the  air.  Would  a  pound  of  feathers 
and  a  pound  of  lead  balance,  if  placed  in  a  vacuum  ?  On  what 
principle  does  a  balloon  rise  ?  What  is  the  amount  of  the  press- 
ure of  the  air?  Describe  the  experiment  illustrating  this. 
Where  do  these  figures  apply? 

Describe  how  the  pressure  of  the  air  continually  varies. 
Explain  Mariotte's  (called  also  Boyle's)  law.  Describe  the  ba- 
rometer. Its  uses.  Are  the  terms  "fair,"  "foul,"  etc.,  often 
placed  on  the  scale,  to  be  relied  upon?  Why  is  mercury  used 
for  filling  the  barometer?  Describe  Otto  Guericke's  barometer. 

Describe  the  action  of  the  lifting-pump.  The  force-pump. 
The  fire-engine.  Compare  the  action  of  the  lifting-pump  with 
that  of  the  air-pump.  What  is  the  siphon?  Explain  its  theory. 

Describe  the  pneumatic  inkstand.  The  hydraulic  ram.  The 
atomizer.  Show  how  a  current  of  air  drags  with  it  the  still 
atmosphere.  What  opposing  forces  act  on  the  air?  How  high 
does  the  air  extend?  How  does  its  density  vary? 

VI.  Acoustics. — Define.  Name  and  define  the  two  senses  of 
this  word.  May  not  the  terms  "light,"  "heat,"  etc.,  be  used  in 
the  same  way  ?  Illustrate  the  formation  of  sound  by  vibrations. 

Show  how  the  sound  of  a  tuning-fork  is  conveyed  through 
the  air.  The  report  of  a  gun.  The  sound  of  a  bell.  The  human 
voice.  Define  a  sound-wave.  In  which  direction  do  the  mole- 
cules of  air  vibrate?  In  what  form  do  the  waves  spread?  Can 
a  sound  be  made  in  a  vacuum?  Can  a  sound  come  to  the  earth 
from  the  stars? 


364  APPENDIX. 

How  do  sounds  change  as  we  pass  above  or  below  the  sea- 
level?  Upon  what  does  the  velocity  of  sound  depend?  Why  is 
this  ?  At  what  rate  does  sound  travel  in  the  air  ?  In  water  ?  In 
the  metals?  In  iron?  What  effect  does  temperature  have  on 
the  velocity  of  sound? 

Do  all  sounds  travel  at  the  same  rate?  How  does  the  ve- 
locity of  sound  enable  us  to  determine  distance?  Upon  what 
does  the  intensity  of  sound  depend?  At  what  rate  does  it 
diminish  ?  Why  ? 

Explain  the  speaking-tube.  The  ear-trumpet.  Describe  Biot's 
experiment  in  the  water-pipes  of  Paris.  The  speaking-trumpet. 
What  is  the  refraction  of  sound? 

Define  reflection  of  sound.  What  is  the  law?  Give  some 
curious  instances  of  reflection.  What  is  the  shape  of  a  whisper- 
ing-gallery ?  Illustrate  the  decrease  of  sound  by  repeated  reflec- 
tion. Why  are  sounds  more  distinct  at  night  than  by  day?  Is 
it  desirable  to  have  a  aoor  or  a  window  behind  a  speaker? 
What  causes  the  "ringing"  of  a  sea-shell?  How  are  echoes 
produced?  When  is  the  echo  repeated?  Illustrate  the  decrease 
of  sound  by  reflection.  What  are  acoustic  clouds?* 

*  "The  influence  of  wind  on  the  intensity  of  sound  seems  due  to  the 
fact  that,  owing  to  obstructions  opposed  by  the  ground,  there  is  a  consider- 
able difference  between  the  velocity  of  the  wind  close  to  the  ground  and 
the  velocity  at  the  height  of  a  few  feet  above  the  ground.  Thus  in  a 
meadow  the  velocity  of  the  wind  at  one  foot  above  the  surface  may  be 
only  half  what  it  is  at  eight  feet  above  the  surface.  Let  us  take  the 
velocity  of  sound  at  1,100  feet  per  second,  and  suppose  that  the  velocity  of 
a  contrary  wind  is  ten  feet  per  second  at  the  surface,  and  twenty  feet  per 
second  at  the  height  of  eight  feet  above  the  surface.  Thus,  considering 
this  circumstance  alone,  the  wave  of  sound  at  the  end  of  a  second  would 
be  at  the  surface  ten  feet  in  advance  of  its  position  at  eight  feet  above  the 
surface ;  so  that  the  front  of  the  wave,  instead  of  being  a  vertical  plane, 
would  be  inclined  to  the  horizon.  Thus  the  sound,  instead  of  proceeding 
horizontally,  becomes  turned  upward.  It  only  remains  to  add  that  this  tilt- 
ing of  the  front  of  the  wave  is  not  delayed  until  the  end  of  a  second,  but 
begins  at  the  origin  of  the  sound  and  increases  gradually.  Hence  a  ray  of 
sound,  so  to  speak,  instead  of  traveling  horizontally  is  curved  upward,  and 
thus  passes  over  the  head  of  a  person  stationed  at  a  distance  from  the 
origin.  A  contrary  wind  then  diminishes  the  intensity  of  sound  by  lifting 
the  sound  off  the  ground,  and  the  amount  of  this  lifting  increases  as  the 
distance  from  the  origin  increases.  The  various  consequences  which  may 
be  deduced  from  the  preceding  theory  have  been  verified  by  experiments. 


QUESTIONS.  365 

What   is    the    difference   between   noise   and   music?     Upon 
what  does  pitch  depend?     Describe  the  siren.     How  is  it  used 

Thus  it  follows  that  a  listener  when  the  wind  is  contrary  may  expect  to 
recover  a  sound,  which  he  has  lost  at  a  certain  distance  from  its  origin,  by 
ascending  to  some  height  above  the  surface.  Also  the  influence  of  a  wind 
will  be  but  small  if  the  surface  be  very  smooth;  thus  sounds  are  heard 
against  the  wind  much  farther  over  calm  water  than  over  land.  Again, 
suppose  the  origin  of  the  sound  to  be  elevated  above  the  surface :  then  if 
the  listener  be  also  raised  above  the  surface  he  may  hear  a  very  loud  sound 
made  up  of  two  parts,  namely,  that  which  has  traveled  horizontally,  and 
that  which  has  been  tilted  upward  from  the  ground  by  the  action  of  the 
contrary  wind.  Next,  suppose  the  wind  to  be  favorable  instead  of  contrary. 
In  this  case  the  higher  part  of  the  wave  of  sound  moves  more  rapidly  than 
the  lower,  and  so  the  plane  front  of  the  wave  is  tilted  forward,  and  the 
rays  of  sound  are  bent  downward  to  the  advantage  of  the  listener  on  the 
ground.  Then  the  influence  of  the  wind  on  sound  has  been  shown  to  de- 
pend on  the  circumstance  that  when  the  wind  is  blowing,  the  velocity  of 
sound  is  different  at  different  heights  above  the  ground ;  similar  effects 
will  therefore  follow  if  this  difference  of  velocity  is  produced  by  any  other 
cause  instead  of  by  the  wind.  Now  change  of  temperature  affects  the  ve- 
locity of  sound :  if  the  temperature  rise  one  degree  of  Fahrenheit's  ther- 
mometer, the  velocity  increases  by  about  a  foot  per  second.  In  general,  as 
we  ascend  in  the  air  during  the  day  the  temperature  decreases,  and  therefore 
so  also  does  the  velocity  of  sound.  Thus  the  result  is  the  same  as  in  the 
case  of  a  contrary  wind ;  the  ray  of  sound  is  lifted  over  the  head  of  a  per- 
son on  the  ground,  so  that  the  audibility  of  the  sound  is  diminished.  The 
presence  of  vapor  in  the  atmosphere  also  affects  the  propagation  of  sound ; 
the  velocity  increases  as  the  quantity  of  vapor  increases.  The  direct  effect, 
however,  is  very  slight,  bub  indirectly  the  vapor  is  of  consequence,  for  it 
gives  to  the  air  a  greater  power  of  radiating  and  absorbing  heat,  and  so 
promotes  inequality  of  temperature.  The  variation  of  temperature  is 
greatest  when  the  sun  is  shining,  so  that  it  is  greater  by  day  than  by 
night,  and  greater  in  summer  than  in  winter.  Hence,  according  to  the 
theory  now  explained,  sounds  ought  to  be  heard*  more  plainly  by  night 
than  by  day,  and  more  plainly  in  winter  than  in  summer.  That  sounds 
are  heard  more  plainly  by  night  than  by  day  is  a  well-known  fact.  We 
have  supposed  that  the  temperature  decreases  as  we  ascend  in  the  atmos- 
phere ;  but  it  may  happen  on  some  occasion  that  the  temperature  at  the 
surface  is  lower  than  it  is  a  little  above  the  surface.  This  may  be  the  case, 
for  instance,  over  the  surface  of  the  sea  in  the  day-time,  and  over  the  sur- 
face of  the  land  by  night.  Thus  the  effect  on  sound  will  be  similar  to  that 
of  a  favorable  wind.  It  is  obvious  that  by  the  combined  influence  of  wind 
and  temperature  the  results  produced  may  vary  much  as  to  degree ;  for 
instance,  the  operation  of  a  contrary  wind  may  be  neutralized  by  that  of 
the  temperature  rising  as  we  ascend  above  the  surface."  See  "Proceedings 
of  the  Royal  Society  of  Great  Britain,"  volumes  XXTL  and  XXIV. 


366  APPENDIX. 

to  determine  the  number  of  vibrations  in  a  sound?  How  is  the 
octave  of  any  note  produced  ?  How  can  we  ascertain  the  length 
of  the  wave  in  sound?  What  length  of  wave  produces  the  low 
tones  in  music?  The  high  tones?  Give  the  illustration  of  the 
locomotive  whistle.  When  are  two  tones  in  unison?  How  can 
we  find  the  length  'of  the  wave  in  any  musical  sound  ?  What  is 
meant  by  the  super-position  of  sound-waves? 

How  can  two  sounds  produce  silence?  What  is  this  effect 
termed?  Illustrate  interference  by  means  of  a  tuning-fork. 
What  are  "beats"?  Describe  the  vibration  of  a  cord. 

Describe  the  sonometer.  What  is  the  object  of  the  wooden 
box?  Give  the  three  laws  of  the  vibration  of  cords.  What  is  a 
node?  Describe  the  experiments  illustrating  the  formation  of 
nodes.  What  are  acoustic  figures?  Nodal  lines? 

What  is  the  fundamental  tone  of  a  cord?  A  harmonic? 
What  causes  the  difference  in  the  sound  of  various  instruments  ? 
Does  a  bell  vibrate  in  nodes?  The  violin-case?  A  piano  sound- 
ing-board ?  State  the  fractions  representing  the  relative  rates  of 
vibration  of  the  different  notes  of  the  scale.  How  is  the  sound 
produced  in  wind-instruments?  How  is  the  sound-wave  started 
in  an  organ-pipe  ?  In  a  flute  ?  What  determines  the  pitch  ? 
What  are  sympathetic  vibrations  ?  Describe  the  resonance  globe. 
What  is  a  sensitive  flame? 

A  singing  flame?  Describe  the  phonograph.  The  ear.  What 
is  the  office  of  the  Eustachian  tube?  Is  there  any  opening;  be- 
tween the  external  and  internal  ear?  What  effect  does  it  have 
on  the  hearing  to  increase  or  diminish  the  pressure  of  the  air? 
How  does  a  concussion  sometimes  cause  temporary  deafness? 
How  can  this  be  remedied?  What  are  the  limits  of  hearing? 
Does  the  range  vary  in  different  persons?  What  sounds  are 
generally  heard  most  acutely?  Are  there  probably  sounds  in 
Nature  we  never  hear?  What  causes  the  "whispering  of  the 
pines  "  ? 

VII.  Optics. — Define.  A  luminous  body.  A  non-luminous 
body.  A  medium.  A  transparent  body.  A  translucent  body. 
An  opaque  body.  A  ray  of  light.  Show  that  neither  air  nor 
water  is  perfectly  transparent.  Why  is  the  sun's  light  fainter 
at  sunset  than  at  midday?  Define  the  visual  angle.  Show  how 
distance  and  size  are  intimately  related. 


QUESTIONS.  867 

State  the  laws  of  light.  Do  they  resemble  those  of  sound  ? 
What  is  the  velocity  of  light?  How  is  this  proved?  Explain 
the  undulatory  theory  of  light. 

How  does  light-motion  differ  from  sound-motion?  What  is 
diffused  light?  Why  are  some  objects  brilliant  and  others  dull? 

Why  can  we  see  a  rough  surface  at  any  angle,  and  an  im- 
age in  the  mirror  at  only  a  particular  one?  Would  a  perfectly 
smooth  mirror  be  visible?  How  does  reflection  vary?  Define 
mirrors.  Name  and  define  the  three  kinds. 

What  is  the  general  principle  of  mirrors  ?  Why  is  an  image 
in  a  plane  mirror  symmetrical?  Why  is  it  reversed  right  and 
left?  Why  is  it  as  far  behind  the  mirror  as  the  object  is 
before  it? 

Why  can  we  often  see  in  a  mirror  several  images  of  an  ob- 
ject? Why  can  we  see  these  best  if  we  look  into  the  mirror 
very  obliquely?  Why  is  an  image  seen  in  water  inverted? 
When  the  moon  is  near  the  meridian,  why  can  we  see  the  im- 
age in  the  water  at  only  one  spot?  When  do  we  see  a  trem- 
ulous line  of  light?  What  is  the  action  of  a  concave  mirror  on 
rays  of  light?  Define  the  focus.  Center  of  curvature.  Focal 
distance.  Describe  the  image  seen  in  a  concave  mirror.  What 
are  conjugate  foci  ?  Describe  the  image  seen  in  a  convex  mir- 
ror. Why  is  it  smaller  than  life?  Why  can  it  not  be  inverted 
like  one  seen  in  a  concave  mirror? 

Define  Refraction.  Does  the  partial  reflection  of  light  as  it 
passes  from  one  medium  to  another  of  different  density  have  a 
parallel  in  sound?  Why  is  powdered  ice  opaque  while  a  block 
of  ice  is  transparent?  Give  illustrations  of  refraction. 

Why  does  an  object  in  water  appear  to  be  above  its  true 
place?  What  is  the  general  principle  of  refraction?  .State  the 
laws  of  refraction.  Explain  total  reflection.  What  is  the  critical 
angle?  Describe  the  path  of  a  ray  through  a  window-glass.  Is 
the  direction  of  objects  changed?  Describe  the  path  through  a 
prism. 

Name  and  describe  the  different  kinds  of  lenses.  What  is 
the  effect  of  a  double-convex  lens  on  rays  of  light?  What  is 
this  kind  of  lens  often  called?  Describe  the  image.  Why  is  it 
inverted  after  we  pass  the  principal  focus  ?  Why  is  it  decreased 
in  size?  What  is  the  effect  of  a  double-concave  lens  on  rays  of 


368  APPENDIX. 

light  ?  Describe  the  image.  Why  can  it  not  be  inverted  like  one 
through  a  double-convex  lens?  Describe  the  images  seen  in  the 
large  vases  in  the  windows  of  drug-stores.  What  is  Aberra- 
tion ?  * 

What  is  mirage?    Give  its  cause. 

How  is  the  solar  spectrum  formed  ?  Name  the  six  principal 
colors.  Show  that  these  six  will  form  white  light.  Why  are 
the  rays  separated?  What  is  meant  by  the  dispersive  power  of 
a  prism?  What  apparatus  possesses  this  property  in  a  high  de- 
gree? Ans.  A  triangular  bottle  filled  with  a  liquid  called  car- 
bon disulphide  ("Popular  Chemistry,"  p.  110).  Why  does  the 
window  of  a  photographer's  dark  room  sometimes  contain  yel- 
low glass? 

Describe  the  three  kinds  of  spectra.  The  spectroscope.  What 
are  its  uses?  Describe  rainbows — primary  and  secondary.  Why 
is  the  rainbow  circular?  How  is  the  rainbow  formed?  Why 
must  it  rain  and  the  sun  shine  at  the  same  time,  to  produce  the 
bow  ?  Why  is  the  bow  in  the  sky  opposite  the  sun  ?  How  many 
refractions  and  reflections  form  the  primary  bow?  The  second- 
ary? How  many  colors  can  one  receive  from  a  single  drop? 
Define  complementary  colors.  How  can  they  be  seen?  What 

*  To  prevent  spherical  aberration  the  pupil  of  the  eye  can  be  made  very 
small.  The  photographer  reaches  the  same  result  by  the  use  of  a  diaphragm 
with  a  small  aperture.  "  The  power  of  a  small  orifice  to  correct  the  greatest 
amount  of  distortion  from  interfering  rays  is  shown  by  a  simple  experi- 
ment. The  normal  eye  of  an  adult  can  not  see  to  read  small  print  nearer 
than  six  inches.  Within  that  distance  the  type  becomes  more  indistinct 
the  closer  it  approaches  the  eye.  But  if  we  make  a  pin-hole  through  a  card 
and  place  it  close  to  the  eye,  we  can  see  to  read  printed  matter  of  any  size 
even  as  near  as  half  an  inch  from  the  eye.  At  that  distance  we  can  see 
even  the  texture  of  fine  cambric  with  microscopic  definition.  The  cause  of 
this  is  easily  explicable.  The  rays  striking  the  lens  perpendicularly  on  the 
center  suffer  no  refraction.  The  effect  of  the  pin-hole  is  to  exclude  aU  rays 
but  those  that  impinge  perpendicularly  on  the  center  of  the  eye  lenses. 
Hence  the  image  of  the  object  close  in  front  of  the  eye  is  pictured  on  the 
retina  without  the  interference  of  the  surrounding  rays,  which  would  fall 
obliquely  on  the  lens,  and  being  refracted  out  of  focus  would  blur  the  pict- 
ure. Observation  of  the  effect  of  a  small  orifice  in  correcting  aberrant  rays, 
and  of  the  fact  that  the  pupil  contracts  in  near  vision,  led  Haller  and  some 
other  physiologists  to  believe  that  contraction  of  the  pupil  was  the  sole  fac- 
tor in  near  accommodation.  But  this  view  has  been  sufficiently  refuted  by 
other  observers."— Dr.  Dudgeon's  "Human  Eye"  p.  76. 


QUESTIONS.  369 

is  the  effect  of  complementary  colors  when  brought  in  contrast  ? 
(In  Fig.  163  opposite  colors  are  complementary.)  Why  do  colors 
seen  by  artificial  light  appear  differently  than  by  daylight — as 
yellow  seems  white,  blue  turns  to  green,  etc. 

Describe  Newton's  rings.  How  are  these  explained  according 
to  the  wave  theory?  What  causes  the  play  of  color  in  mother- 
of-pearl  ?  In  soap-bubbles  ?  In  the  scum  on  stagnant  water  ?  In 
thin  layers  of  mica  or  quartz  ? 

What  can  you  say  about  the  length  of  the  waves  ?  State  the 
analogy  between  color  and  pitch  in  music.  Why  is  grass  green  ? 
When  is  a  body  white?  Black?  What  is  color-blindness? 

What  is  double  refraction?  What  are  the  two  rays  termed? 
What  is  polarized  light?  How  does  a  dot  appear  through  Ice- 
land spar?  What  other  methods  are  there  of  polarizing  light? 
State  some  illustrations  and  practical  uses  of  polarized  light. 

What  is  the  meaning  of  the  word  microscope?  Describe  the 
simple  microscope.  The  compound  microscope.  How  is  the 
power  of  a  microscope  indicated  ?  Do  we  see  the  object  directly 
in  a  microscope?  Why  is  the  object-lens  made  so  small  and  so 
convex  ? 

What  is  the  meaning  of  the  word  telescope?  Describe  the 
reflecting  telescope.  The  refracting  telescope.  What  is  the  use 
of  the  object-lens  ?  The  eye-piece  ?  Is  the  image  inverted  ?  De- 
scribe the  opera-glass. 

The  stereoscope.  The  projecting  lantern.  How  are  dissolv- 
ing views  produced? 

Describe  the  Camera.     The  structure  of  the  eye.*     The  for- 

*  "  In  the  skate's  eye,  and  generally  in  the  eyes  of  fishes,  the  cornea  is 
nearly  flat,  the  aqueous  humor  is  insignificant,  and  there  is  virtually  no  an- 
terior chamber,  for  the  crystalline  lens  comes  up  close  to  the  cornea.  A 
convex  cornea  filled  by  an  aqueous  humor  would  be  of  no  use  in  the  water, 
the  refractive  index  of  the  water  being  identical  with  that  of  the  aqueous 
humor.  Accordingly,  the  refraction  of  the  rays  of  light  has  to  be  effected 
entirely  by  the  crystalline  lens,  which  is  nearly  spherical,  and  of  much 
greater  refractive  power  than  the  corresponding  organ  in  animals  which 
pass  their  lives  in  the  air.  The  crystalline  lens  being  so  nearly  spherical  in 
shape  and  of  such  high  refractive  power,  the  axis  of  the  eye  is  short.  The 
eye  of  the  turtle,  which  is  so  much  in  the  water,  is  very  similar  to  that  of 
the  fish.  The  crystalline  lens  is  very  near  the  cornea.  The  lens  is  smaller 
proportionally  than  that  of  the  skate,  nor  is  it  nearly  so  spherical ;  and  its 


370  APPENDIX. 

mation  of  an  image  on  the  retina.  The  adjustment  of  the  eye. 
The  cause  of  near  and  over  sightedness.  The  remedy.  Why  do 
old  people  hold  a  book  at  arm's  length?  Illustrate  the  duration 
of  an  impression.  What  is  the  range  of  the  eye? 

VIII.  Heat.— Define  solar  energy.  In  what  ways  may  it  be- 
come manifested?  What  is  a  diathermanous  body?  Cold? 
Gases  and  vapors?  Show  the  intimate  relation  between  light 
and  heat.  What  is  light?  What  is  the  theory  of  heat?  Why 
can  we  not  see  with  our  fingers  or  taste  with  our  ears?  At 
what  rate  does  nerve-motion  travel?  (See  "Hygienic  Physiol- 
ogy," p.  177.)  How  long  does  it  take  a  man,  six  feet  in  height, 
to  find  out  what  is  going  on  in  his  foot? 

Name  the  sources  of  heat.  Describe  and  illustrate  each  of 
these.  Can  force  be  destroyed?  If  apparently  lost,  what  be- 
comes of  it?  What  is  Joule's  law?  Define  latent,  sensible,  and 
specific  heat.  Explain  the  paradox,  "  that  freezing  is  a  warm- 
ing process  and  thawing  a  cooling  one."  Why  does  "heat  ex- 
pand and  cold  contract"?  What  do  you  say  as  to  the  expan- 
sion of  solids,  liquids,  and  gases?  Illustrate  the  expansion  of 
solids.  Is  it  better  to  buy  alcohol  in  summer  or  in  winter? 
What  is  the  thermometer?  Describe  it.  Describe  the  process 
of  filling  and  grading.  The  F.,  C.,  and  B.  scales.  Tell  what 
you  can  about  liquefaction.  Of  a  solid.  Of  a  gas.  In  one 
case  sensible  heat  becomes  latent,  in  the  other  latent  heat  be- 
comes sensible — why  is  this? 

Explain  how  a  freezing  mixture  "makes  ice-cream."  State 
the  theory  of  vaporization.  Of  distillation.  Since  rain  comes 
from  the  ocean,  why  is  it  not  salt?  Describe  the  theory  of 
boiling.  What  is  the  boiling-point?  Do  all  liquids  boil  at  the 
same  temperature  ?  What  would  be  the  effect,  if  this  were  the 

density,  and  consequently  its  refractive  power,  is  somewhat  less.  Hence  it 
has  proportionally  a  longer  focus.  The  cornea  is  more  convex  than  that  of 
the  skate.  The  fish  having  no  eyelids  nor  any  lachrymal  apparatus,  its  cor- 
nea will  be  apt  to  become  dim  by  exposure  to  the  air,  but  the  turtle  is  well 
supplied  with  the  requisite  apparatus  for  maintaining  the  transparency  of 
the  eye  in  air.  Ophidian  reptiles  have  no  eyelids  or  lachrymal  apparatus, 
but  they  do  not  require  them,  as  their  cornea  is  transparent  though  dry." 
— Dr.  Dudgeon's  '•'•Human  Eye,"  p.  50. 


QUESTIONS.  371 

case?  Upon  what  does  the  boiling-point  depend?  Why  does 
pressure  raise  the  melting-point  of  most  substances  but  lessen 
that  of  ice?  Why  does  salt-water  boil  at  a  higher  temperature 
than  f ruuh-water ?  Why  will  milk  boil  over  so  easily?  Why 
will  soup  keep  hot  longer  than  boiling  water?  Does  the  air, 
dissolved  in  water,  have  any  influence  on  the  boiling-point? 
Can  you  measure  the  height  of  a  mountain  by  means  of  a  tea- 
kettle and  a  thermometer?  Show  how  cold  water  may  be  used 
to  make  warm  water  boil.  At  what  temperature  will  water 
boil  in  a  vacuum?  Why?  Can  we  heat  water  in  the  open  air 
above  the  boiling-point?  What  becomes  of  the  extra  heat? 
What  is  the  latent  heat  of  water?  Upon  what  principle  are 
buildings  heated  by  steam?  Have  you  ever  seen  any  steam? 

Define  evaporation.  Does  snow  evaporate  in  the  winter? 
What  can  be  done  to  hasten  evaporation?  Why  is  a  saucepan 
made  broad  ?  Why  do  we  cool  ourselves  by  fanning  ?  Why  does 
an  application  of  spirits  to  the  forehead  allay  fever  ?  Why  does 
wind  hasten  the  drying  of  clothes?  Describe  a  vacuum-pan. 
Why  is  evaporation  hastened  in  a  vacuum?  Why  is  evapora- 
tion a  cooling  process  ?  How  is  ice  manufactured  in  the  tropics  ? 
What  is  the  spheroidal. state? 

Name  and  define  the  three  modes  of  communicating  heat. 
Give  illustrations  showing  the  relative  conducting  power  of 
solids,  liquids,  and  gases.  What  substances  are  the  best  con- 
ductors? Is  water  a  good  conductor?  Air?  What  is  the 
principle  of  ice-houses?  Fire-proof  safes?  Why  do  not  flannel 
and  marble  appear  to  be  of  the  same  temperature?  Is  ice 
always  of  the  same  temperature?  Describe  the  convective  cur- 
rents in  heating  water.  Where  must  tfhe  heat  be  applied? 
Where  should  ice  be  applied  in  order  to  cool  water?  Describe 
the  convective  currents  in  heating  air.  Upon  what  principle 
are  hot-air  furnaces  constructed?  Ought  the  ventilator  at  the 
top  of  a  room  to  be  opened  in  winter?  At  the  bottom?  Is 
interplanetary  space  warmed  by  the  sunbeam? 

Does  the  heat  of  the  sun  come  in  through  our  windows? 
Does  the  heat  of  our  stoves  pass  out  in  the  same  way?  Show 
how  the  vapor  in  the  air  helps  to  keep  the  earth  warm.  Ex- 
plain the  Radiometer.  The  relation  between  absorption  and 
reflection. 


372  APPENDIX. 

What  is  the  elastic  force  of  steam  at  the  ordinary  press- 
ure of  the  air?  What  is  the  difference  between  a  high-press- 
ure and  a  low-pressure  engine?  Which  is  used  for  a  locomo- 
tive? Why?  Describe  the  governor.  What  is  the  object  of 
a  fly-wheel? 

How  does  the  capacity  of  the  air  for  moisture  vary?  What 
is  the  principle  on  which  dew,  rain,  etc.,  depend?  Show  that 
a  change  in  density  produces  a  change  in  temperature.  What 
effect  does  this  have  on  the  temperature  of  elevated  regions? 
Does  an  ounce  of  air  on  a  mountain-top  contain  the  same 
quantity  of  heat  as  the  same  weight  at  the  foot?  How  is  dew 
formed  ? 

Upon  what  objects  will  it  collect  most  readily?  Why  will 
it  not  form  on  windy  nights?  Why  is  rice-straw  used  in  Ben- 
gal in  making  ice?  What  is  a  fog?  Why  do  fogs  form  over 
ponds  in  the  early  evening?  Cause  of  fogs  over  the  Newfound- 
land banks?  How  does  a  fog  differ  from  a  cloud?  Why  do 
clouds  remain  suspended  in  the  air?  Describe  the  different 
kinds  of  clouds.  Describe  the  formation  of  rain.  Snow. 

How  are  winds  produced?  Land-and-sea  breezes?  Trade- 
winds?  Oceanic  currents?  Tell  about  the  Gulf  Stream.  Ex- 
plain the  influence  which  water  has  on  climate.  Of  what  prac- 
tical use  is  the  air  in  water?  Describe  the  apparent  exception 
which  exists  in  the  freezing  of  water.  Describe  the  two  proc- 
esses by  which  pure  water  can  be  obtained.  How  is  an  ex- 
cessive deposit  of  dew  prevented? 

IX.  Magnetism. — Define  Magnetism.  A  Magnet.  A  natural 
magnet.  An  artificial  one.  A  bar-magnet.  A  horseshoe 
magnet.  The  poles.  The  magnetic  curves.  Describe  a  mag- 
netic needle.  What  is  the  law  of  magnetic  attraction  and  re- 
pulsion? Define  magnetic  induction.  Explain  it. 

When  is  a  body  polarized?  Give  some  illustrations  of  in- 
duced magnetism.  Does  a  magnet  lose  any  force  by  induction? 
How  do  you  explain  the  fact  that  if  you  break  a  magnet  each 
part  will  have  its  N.  and  S.  poles? 

Describe  the  process  of  making  a  magnet.  On  what  prin- 
ciple will  you  explain  this?  Describe  the  compass.  Is  the 
needle  true  to  the  pole?  What  causes  it  to  vary?  What  is  the 


QUESTIONS.  373 

line  of  no  variation?  Declination?  Why  does  the  needle  point 
N.  and  S. ?  What  is  a  dipping-needle?  Explain.  How  is  a 
needle  balanced? 

Where  is  the  N.  magnetic  pole?  How  would  one  know 
when  he  reached  it  ?  Does  the  earth  induce  magnetism  ?  Which 
end  of  an  upright  bar,  in  the  United  States,  will  be  the  S. 
pole? 

X.— Electricity.  Define  frictional  electricity.  The  electro- 
scope. Difference  between  static  and  dynamic  electricity.  Show 
the  existence  of  two  manifestations  of  electricity.  Give  the 
names  applied  to  each. 

State  the  law.  What  is  the  theory  of  electricity?  Define  a 
conductor.  An  insulator. 

What  is  the  best  conductor?  Best  insulator?  Is  a  poor  con- 
ductor a  good  insulator?  When  is  a  body  said  to  be  insulated? 
Can  electricity  be  collected  from  an  iron  rod?  Describe  a  plate- 
glass  electrical  machine.  What  is  the  use  of  the  chain  at  the 
negative  pole?  Define  electrical  induction.  State  Faraday's 
theory. 

What  is  the  relation  between  induction  and  attraction  and 
repulsion?  Describe  the  electric  chime.  Explain.  Describe  the 
dancing  images.  The  Leyden  jar.  What  gives  the  color  to  the 
spark?  How  is  the  jar  discharged? 

What  are  the  essentials  of  a  Leyden  jar?  What  is  the  ob- 
ject of  the  glass?  The  tin-foil?  State  the  theory  of  the  charg- 
ing of  the  jar.  Can  an  insulated  jar  be  charged?  Is  the  elec- 
tricity on  the  surface  or  in  the  glass?  Can  the  inner  molecules 
of  a  solid  conductor  be  charged?  Will  a  rod  contain  any  more 
electricity  than  a  tube?  Why  is  the  prime  conductor  of  an 
electrical  machine  hollow?  What  is  the  effect  of  points?  De- 
scribe the  electric  whirl.  Explain  the  existence  of  electricity  in 
the  atmosphere.  What  is  the  cause  of  lightning?  Thunder?  Is 
there  any  danger  after  you  once  hear  the  report?  Describe  the 
different  kinds  of  lightning.  Tell  how  Franklin  discovered  the 
identity  of  lightning  and  frictional  electricity. 

Tell  what  you  can  about  lightning-rods.  In  what  consists  the 
main  value  of  the  rod?  Does  the  lightning  ever  pass  upward 
from  the  earth?  Ans.  It  does,  both  quietly  and  by  sudden 


374  APPENDIX. 

discharge.  Has  nature  provided  any  lightning-rods?  What  is 
St.  Elmo's  fire?  What  is  the  velocity  of  electricity?  Illustrate 
its  instantaneousness.  Explain  the  action  of  the  Voss  elec- 
trical machine. 

Name  some  of  the  effects  of  Motional  electricity — (1)  Phys- 
ical, (2)  Chemical,  (3)  Physiological.  How  are  voltaic  electricity 
and  chemistry  related?  Why  is  voltaic  electricity  thus  named? 
Tell  the  story  of  Galvani's  discovery.  What  was  his  theory? 
Give  an  account  of  Volta's  discovery.  How  can  we  form  a 
simple  pile?  Describe  the  simple  voltaic  circuit. 

Define  the  poles.  Electrodes.  Closing  and  breaking  the  cir- 
cuit. What  is  necessary  to  form  a  voltaic  pair?  Describe  the 
chemical  change.  Why  does  the  hydrogen  come  off  from  the 
copper?  Tell  what  you  can  about  the  current. 

What  really  passes  along  the  wire  ?  How  is  this  force  trans- 
mitted ?  Will  a  tube,  then,  convey  as  much  electricity  as  a  rod  ? 
Explain  the  term  electric  potential. 

Describe  Smee's  battery.  Grove's  battery.  The  chemical 
change  in  this  battery.  What  are  the  advantages  of  Grove's 
battery?  Describe  Bunsen's  battery.  Daniell's  battery.  The 
Potassium  Bichromate  battery.  Compare  frictional  and  voltaic 
electricity.  i 

State  the  effects  of  voltaic  electricity,  (1)  Physical — heat  and 
light ;  (2)  Chemical — decomposition  of  water,  electrolysis,  electro- 
typing,  electro-plating,  etc.;  (3)  Physiological. 

What  is  the  effect  of  a  voltaic  current  on  a  magnetic  needle  ? 
What  is  a  galvanometer?  An  electro-magnet?  Show  how  a 
coll  can  be  magnetized.  How  are  bar-magnets  made  ?  Describe 
the  magnetic  telegraph.  How  is  a  message  sent?  How  is  one 
received?  What  is  a  sounder?  What  is  the  general  principle 
of  the  telegraph?  Describe  the  relay.  Name  the  use  of  each 
instrument.  Describe  a  magneto-electric  machine.  Describe 
Wilde's  machine.  What  are  induced  currents?  Describe  the 
Telephone.  The  Microphone.  What  is  the  difference  between 
the  acoustic  and  the  magnetic  telephone?  Explain  Ruhmkorff's 
coil.  Thermal  electricity.  A  thermo-electric  pile.  Describe  the 
electric  fish. 


INDEX. 


The  index   figures   denote  the  page. 


Aberration,  208,  219. 
Achromatic,  220. 
Acoustics,  153. 
Acoustic  clouds,  165. 
figures,  174. 

Action  and  reaction,  26. 
Adhesion,  49. 
Air,  129. 
Air-pump,  129. 
Alcoholmeter,  116. 
Amplitude,  68. 
Analyzer,  224. 
Annealing,  47. 

Archimedes,  97,  135,  149,  239. 
Law  of,  115,  149. 
Aristotle,  15,  98,  277. 
Artesian  wells,  111. 
Atmosphere,  136,  145. 
Atomic  theory,  3. 
Atomizer,  143. 
Attraction,  41. 

"  of  adhesion,  49. 

*'  of  cohesion,  43. 

"  Capillary,  49. 

Gravitation,  55. 
Avogadro's  law,  16. 


Bacon,  15,  277. 
Barker's  null,  125. 
Barometer,  138. 
Battery,  Bunsen's,  320. 

"        Grove's,  320. 

u        Daniell's,  319. 


Battery,  Potassium  Bichromate,  319. 

Thermo-electric,  346. 
Beats,  171,  186. 
Bell,  156,  175. 
Boiling,  252. 
Bolometer,  347. 
Brittleness,  14. 
Britannia  Bridge,  105. 


Caloric,  277. 
Camera,  230. 
Capillarity,  49. 
Capstan,  87. 
Cartesian  diver,  131. 
Caustic,  199. 
Center  of  gravity,  57. 

"       "  oscillation,  69. 

**       "  percussion,  70. 
Centrifugal  force,  31. 
Chemical  affinity,  43. 

"         change,  4. 
Chromatic  aberration,  219. 
Clepsydra,  78. 
Clock,  71. 
Clouds,  266. 
Cohesion,  43. 
Coils,  Induction,  337. 
Color,  216. 

"      blindness,  216. 

"      Complementary,  216. 
Columns  of  air,  177. 
Compass,  287. 

Compensation  pendulum,  248. 
Condenser,  130. 


376 


INDEX. 


Conductors,  299. 
Conservation  of  energy,  35. 
Cords,  Vibration  of,  171. 
Correlation  of  forces,  278. 
Co-vibration,  158,  179. 
Crystals,  46. 

Cumulative  contrivances,  93. 
Current,  Electric,  315. 
of  rivers,  123. 
Curves,  Magnetic,  285. 


Declination,  285. 
Democritus,  16,  277 
Dew,  265. 

Diathermancy,  275. 
Dichroic,  217. 
Diffraction,  221. 
Diffusion  of  liquids,  52. 

"   gases,  52. 
Dissolving  views,  229. 
Distillation,  251. 
Divisibility,  8. 
Double  refraction,  222. 
Ductility,  10. 
Dynamo-electric  machine,  343. 

E 

Ear,  The,  181. 
Ear  of  Dionysius,  185. 
Ear-trumpet,  161. 
Echoes,  164. 
Elasticity,  12. 
Electric  battery,  318. 
chime,  304. 
light,  345. 
"        potential,  297. 
"        telegraph,  330. 

whirl,  300. 

Electrical  machine,  Plate,  302. 
Voss,  308. 
Electricity,  292. 

Animal,  347. 
Frictional,  294. 
Voltaic,  313. 
Electrophorus,  296. 
Electro-gilding,  325. 


Electro-magnets,  330. 

"       negative   and    positive   sub- 
stances, 323. 

"       plating,  325. 
Electrolysis,  322. 
Electromotive  force,  316. 
Electroscope,  294. 
Energy,  4,  34. 

Kinetic,  35,  65. 

"         Potential,  35. 
Radiant,  243. 

"         Solar,  212. 
Equilibrium,  65. 
Eustachian  tube,  182. 
Evaporation,  254. 
Expansion,  247. 
Extraordinary  ray,  223, 
Extension,  6.  • 

Eye,  The,  230. 


Falling  bodies,  58. 
Faraday,  40,  279,  300. 
Fire-engine,  140. 
Fish,  119. 
Flames,  Sensitive,  179. 

Singing,  180.       « 
Floating  bodies,  118. 
Fly-wheel,  93. 
Focus,  198,  205. 
Fogs,  265. 
Force,  21. 

"       pump,  140. 

"       Centrifugal,  31. 
Centripetal,  31. 

"       Molecular,  43. 
Forces,  Parallelogram  of,  27. 
Triangle  of,  28. 

"        Polygon  of,  28. 

"        Composition  of,  27. 

"        Resolution  of,  28. 
Foucault,  72. 
Fountains,  110. 
Franklin,  310. 
Fraunhofer's  lines,  214. 
Freezing  mixture,  250. 
"         of  water,  271. 


INDEX. 


377 


Friction,  22. 
Frost,  265. 
Fulcrum,  82. 


Galvanometer,  329. 
Galileo,  40,  77,  98,  149,  187. 
Gases,  44. 

Adhesion  of,  52,  144. 

"       Buoyancy  of,  135. 

"       Compressibility  of,  13,  137. 

"      .Diffusion  of,  52. 

"       Elasticity  of,  13,  137. 

"       Osmose  of,  53. 

"       Pressure  of,  133. 
Geissler's  tubes,  339. 
Gold-leaf,  Making  of,  11. 
Governor,  The,  262. 
Gravitation,  55. 
Gravity,  56. 

Center  of,  57. 
Specific,  113. 
Guericke,  134,  138. 
Gulf  Stream,  270. 

H 

Halos,  219. 
Hardness,  14. 
Harmonics,  175. 
Hay-scales,  86. 
Heat,  243. 

"      affected  by  rarefaction,  264. 

"      Absorption  of,  260. 

"      Conduction  of,  257. 

"      Convection  of,  258. 

"      Expansion  by,  247. 

"      Latent,  250. 

"      Mechanical  equivalent  of,  246. 

"      Physical  effects  of,  247. 

"      Radiation  of,  258. 

u      Reflection  of,  260. 

"      Specific,  256. 

"      Theory  of,  244. 

"      unit,  249. 
Heating  by  steam,  270. 
Helix,  328. 
Helmholtz,  188. 


Hiero's  fountain,  133. 
Holtz's  machine,  308. 
Horse-power,  A,  95. 
Huygens,  40,  70,  240. 
Hydrodynamics,  121. 
Hydraulic  ram,  142. 
Hydrometer,  116. 
Hydrostatics,  101. 
Hydrostatic  bellows,  108. 

"  paradox,  109. 

"'          press,  104. 

I 

Ice-crystals,  46,  268. 
Iceland  spar,  222. 
Imbibition,  50. 
Impenetrability,  7. 
Inclined  plane,  88. 
Indestructibility,  10. 
Index  of  refraction,  204. 
Induction,  212,  223. 
Inertia,  23. 
Insulators,  219. 
Interference,  128,  169.1 
Isochronous,  68. 


Joule's  law,  246. 

K 

Kaleidoscope,  197. 
Kite,  30. 


Lantern,  228. 
Le  Conte,  278. 
Lenses,  205. 

Land-and-sea  breeze,  269. 
Lever,  82. 
Leyden  jar,  225. 
Light,  191. 

"       Composition  of,  210. 

"       Diffraction  of,  221. 

"       Interference  of,  220. 

"       Laws  of,  192. 

"       Polarization  of,  221. 

"      Reflection  of,  194. 


378 


INDEX. 


Light,  Refraction  of,  202. 
"       Theory  of,  193. 
"       Total  reflection  of,  208. 
"       Velocity  of,  192. 
"       Waves  of,  193. 
Lightning,  310. 
Lines  of  Force,  285. 
Liquids,  Buoyancy  of   118. 
"         Cohesion  of,  43,  44. 
"        Compressibility  of,  13. 
"        Diffusion  of, .52. 
"        Elasticity  of,  13. 
"        Osmose  of,  53. 
"        Pressure  of,  105. 
"        Specific  gravity  of,  116. 
"         Surface  tension  of,  45. 
"        tend  to  spheres,  45. 
Liquefaction,  250. 

of  gases,  276. 
Lissajous,  188. 
Locke,  277. 

M 

Machinery,  81. 

Magdeburg  hemispheres,  133. 

Magnetic  curves,  285. 

Magnetism,  281. 

Magneto-electric  machine,  342. 

Magneto-induction,  339. 

Magnets,  281. 

Malleability,  11. 

Mariotte's  law,  148. 

Mass,  14,  23. 

Measure,  Standards  of,  16,  114. 

Mechanical  powers,  81. 

Mechanics,  Principle  of,  81. 

Meteorology,  264. 

Meter,  6. 

Metric  system,  17. 

Microphone,  341. 

Microscopes,  225. 

Mirage,  209. 

Mirrors,  194. 

Molecules,  3. 

Molecular  forces,  43. 

Moment  of  a  force,  83. 

Momentum,  23. 


Motion,  21. 

"        Circular,  30. 

"        Communication  of,  21. 

"        Composition  of,  27. 

"        Laws  of,  22. 

"        Perpetual,  94. 

"        Reflection  of,  33. 

"        Resistance  to,  22. 
Multiple  images,  196. 
Music,  165. 
Musical  scale,  176. 

N 

Near-sightedness,  232. 
Needle,  Magnetic,  285. 

Dipping,  288. 
Newton,  40,  77,  239. 
Newton's  rings,  220. 
Nicol's  prism,  224. 
Nodal  lines,  174. 
Nodes,  173. 
Noise,  165. 


Ocean  currents,  269. 
Octave,  168. 
Opera-glass,  228. 
Optics,  191. 

Optical  instruments,  225. 
Ordinary  ray,  223. 
Organ  pipes,  178. 
OsciUation,  Center  of,  69. 
Osmose  of  gases,  53. 

"  liquids,  53. 
Oversightedness,  232. 
Overtones,  175. 


Pascal,  103,  150. 
Pendulum,  68. 
Percussion,  Center  of,  71« 
Perpetual  motion,  94. 
Phonograph,  180,  188. 
Pinion,  88. 
Pisa,  Tower  of,  67. 
Pitch,  166. 
Platinum  wire,  11. 


INDEX. 


379 


Plato,  15. 
Plumb-line,  56. 
Pneumatics,  129. 
Pneumatic  inkstand,  142. 
Polariscope,  223. 
Polarization  of  light,  221. 

"  Electric,  301,  318. 

Magnetic,  284. 
Polarizing  angle,  223. 
Porosity,  8. 
Pressure  of  air,  133. 
Prince  Rupert's  drop,  48. 
Prisms,  204. 
Projecting  lantern,  228. 
Pulley,  91. 
Pumps,  139. 

Air,  129. 

"        Force,  140. 

"        lifting,  139. 

"        Sprengel's  air,  148. 
Pythagoras,  186. 


Radiometer,  258. 
Rain,  267. 
Rainbow,  217. 
Reaction,  25. 
Reaction-wheel,  125. 
Reflected  motion,  33. 
Reflection,  Total,  208. 
Refraction,  Index  of,  204. 
Relay,  334. 
Resonance,  179. 
Rivers,  123. 
Ruhmkorff's  coil,  337. 
Rumford,  Count,  278. 
Rupert's  drop,  48. 

S 

St.  Elmo's  fire,  311. 
Screw,  90. 

Sensitive  flames,  179. 
Ship,  Sailing  of,  29. 
Singing  flames,  179. 
Siphon,  141. 
Siren,  166. 
Size,  14. 


Snow,  267. 
Solution,  51. 
Sonometer,  172. 
Sound,  153. 

"       Intensity  of,  160. 

"       in  a  vacuum,  156. 

"       Interference  of,  169. 

"       Ixmdness  of,  160. 

"       Production  of,  153. 

"       Reflection  of,  162. 

"       Refraction  of,  161. 

"       Transmission  of,  154. 

"       Velocity  of,  158. 
Sounding-boards,  172. 
Sound-waves,  154. 
Speaking-tubes,  161. 

"         trumpet,  161. 
Specific  gravity,  113. 

flask,  116. 
Spectroscope,  213. 
Spectrum,  Prismatic,  210. 
Normal,  212. 
Interruptions  in,  213. 
Kinds  of,  214. 
Analysis,  214. 
Spherical  aberration,  208. 
Spheroidal  state,  256. 
Steam,  252. 

"        engine,  261. 
Steelyard,  85. 
Stereoscope,  234. 
Stringed  instruments,  172. 
Surface  tension,  45. 


Tacking,  30. 
Tackle-block,  93. 
Telegraph,  330. 
Telephone,  340. 
Telescope,  226. 
Temperature,  248. 
Tempering,  47. 
Tenacity,  12. 
Thermo-electricity,  346. 
Thermometers,  248. 
Thunder,  310 
Torricelli,  135,  149. 


380 


INDEX. 


Torsion  pendulum,  13. 
Total  reflection,  208. 
Tourmaline,  222. 
Trade-wind,  269. 
Tubes,  122. 
Turbine  wheel,  124. 
Tyndall,  188. 


Vaporization,  251. 
Velocity,  21. 
Velocity  of  heat,  243. 
"   light,  192. 
"   sound,  158. 
Vibration,  68. 
Vibrations  of  air,  154. 

"   cords,  171. 
"   ether,  193. 
u  "  pendulum,  68. 

"  Sympathetic,  179. 

Virtual  velocity,  98. 
Vision,  232. 

"        Binocular,  233. 
Visual  angle,  191. 
Vocal  Memnon,  185. 
Voltaic  arc,  321. 
"       battery,  318. 


Voltaic  electricity,  313. 
"        pair,  The,  314. 
Volume,  6. 
Voss'  machine,  308. 

W 

Watches,  78. 
Water,  270. 

"       barometer,  138. 

level,  112. 

wheels,  123. 
Waves,  126,  154. 
Wave  motion,  126. 
Wedge,  91. 
Weight,  57,  58. 
Welding,  44. 
Wells,  111. 
Wheel  and  axle,  86. 
Wheel-work,  88. 
Whirligig,  125. 
Wilde's  machine,  343. 
Winds,  268. 
Wind  instruments,  177. 


Young,  240. 
Youmans,  278. 


YB  360C4 


M187542 


THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 


